The pseudoscience that had the most dangerous results were probably the Trofim Lysenko's agricultural recommendations. They played a major role in the Great Chinese Famine of the late 1950s and early 1960s. Anti-nuclear movement and global warming denial are similar in their logic to Lysenkoism, and can easily end up being about as dangerous, if not even more.

My Experience with Pseudosciences

Pseudoscience simply means "false science", something which is presented as science, but demonstrably doesn't follow the methods of science. Many people believe that pseudoscience is basically isolated to specific and widely discredited fields, like astrology and numerology. However, I would argue that pseudoscience is probably everywhere,
a 2 b 2
Do you know how,
for example,
the mathematicians
know the Pythagorean
Theorem is correct?
In school,
we aren't taught
how scientists
know what they know.
at least wherever there are some people fighting for the truth (because at least some of them will be too ignorant for that).
To understand why, consider the case of SCIgen. It's a computer program that generates nonsense in the form of texts about informatics, using the vocabulary typical for them. Those texts were sent to a few well-known peer-reviewed journals. Most of them rejected them, yet some of them accepted them. When asked why, one of the recensents responded that it didn't make sense to him either, but he attributed its apparent incoherence to his own lack of knowledge in that particular field of informatics (Here is a paper I've recently written about informatics, I can understand how somebody might have a hard time differentiating stuff like that from gibberish.).
Pseudosciences are widespread because of the way people react to nonsense: they ascribe the apparent incoherence to their own lack of knowledge. And more intelligent you are, more likely you are to fall into that trap, because, more intelligent somebody is, easier it is for him to imagine that there is something he is unaware of, so that something that doesn't make sense to him actually does make sense. This type of thinking is as flawed as it can get, but it's very common (and perhaps mostly subconscious). In fact, I would argue there are reasons why intelligent but ignorant people are sometimes more likely to believe pseudoscience than actual science. First, pseudoscientists usually have more exciting ideas than actual science does. For example, pseudoscientists often claim to have discovered a method to reliably tell if somebody is lying to you. Those things interest people. Psychologists arguing such methods can't work much better than chance? Not so much. Pseudoscientists often claim they've reconstructed the first human language. Many people find that exciting. They will rather read that than read the linguists explaining their methods are contrary to the methods accepted in linguistics. Second, pseudoscientists usually have shorter rhetorics than the actual science does. For example, pseudoscientists often say things such as that the double-slit experiment proves that bilocation is possible or that the quantum entanglement explains the telekinesis. Now, if people actually read both sides of the story, that is, both what the pseudoscientists write and the actual science of the double-slits experiment and quantum entanglement, they would ask what drugs those pseudoscientists are on. But most of the people don't do that. They think that if someone has a short rhetoric regarding something, he knows what he is talking about. Third, though I am not sure if that plays a significant role, sometimes what makes sense to a scientist is different from what makes sense to a layman. To understand why, consider the following statements: Those statements are false and probably incoherent with what the layman already knows. The metal of the same temperature will cause the ice to melt faster than the wood will for the exactly same reason it feels colder when we touch it. The metal at a room temperature is significantly colder than our body is, and, when we touch it, it takes a lot of heat from our body. A piece of wood, being the same temperature, takes less heat because it's a much poorer heat conductor. A better heat conductor will, of course, melt the ice faster. The second statement is false, because, well, the basic trigonometry. First of all, if the Earth were flat, I hope you agree, there would be no horizon in the first place for us to make this observation. Therefore, simply yelling that the Earth is flat isn't an explanation at all. Of course, those who claim the Earth is flat will claim there is some basically undetectable (for whatever reason) curving of light upwards, and that also can't be considered an explanation.
The horizon rising as we climb diagram.
A diagram explaining
why the horizon
appearing to rise
as we climb doesn't
prove the Earth
is flat
Second, if you draw a diagram, you will see that the angle at which you see the horizon is given by the formula (assuming the Earth is a perfect sphere, which it actually isn't, but that's not so important here) angle=90-arcsin(r/(r+h)), where r is the radius of the Earth (around 6'000'000 meters) and h is your height relative to the surface of the Earth. So, if you are on the Mount Everest (9'000 meters height), you will see the horizon at the angle of 3.14 degrees. Barely perceptible. Actually, it's not even barely perceptible, because, when you are on the Mount Everest, the mountains hide the horizon in the first place. The third statement is wrong, because, well, the word *vīlla (with a long 'i'), borrowed into Old Croatian, would give something like *Bȁo (with a short falling tone on 'a') in Modern Croatian. Even if a linguist doesn't know the sound changes that happened between Old Croatian and Modern Croatian, he still wouldn't have expected the word to remain unchanged for more than a thousand years. The consonants, the vowels and the accent "matching perfectly" (ignoring the sound changes that have occured) is a strong (though not a necessary) argument for the words being unrelated, rather than related, because the sound changes don't affect individual words, but whole languages. A failure of a sound change to affect a word it could have affected is what requires an explanation, and not the word changing to the unrecognizability due to the regular sound changes. The point is, it's very easy for a layman to convince himself those statements (and the similar ones) make sense, sometimes even easier than it is to understand the true science to the point when it actually makes sense. Our senses are unreliable. We witness illusions and hallucinations (at least the ones such as dreams) all the time. Even when they do give us correct information, it's very easy to misinterpret. The science seeks to correct those things. Pseudoscientists sometimes appear to exploit those things to make people believe absurdities.
So, what do I think, what's a good way of recognizing pseudosciences? Well, let me be clear: the knowledge we are taught at school helps very little, if at all. We usually aren't taught how scientists know what they know, and that, during the history of science, most of the supposed discoveries were actually false.
In school, what we are taught about history of science are, at least the vast majority of them, the succes stories. That's not what's going on in the real world, for every discovery in some field, there were countless people who put a lot of effort and were confident they were right, but they got things wildly wrong. Because such things are only rarely mentioned in school, schools make us very susceptible to the survivorship bias, making us think that, if somebody claims to have discovered something new, they likely have. The truth is, somebody who claims to have deciphered some ancient language is more likely to be somebody like Johannes Goropius Becanus, a man who convinced himself and many others that the Egyptian hieroglyphs represent the Dutch language (seriously!), than like Champollion. If most of the experts in the field think you are wrong, and that's whenever you think you've discovered something new, you usually are wrong.
What we are taught at school might even have a detrimental effect to our ability to detect pseudoscience not only because they make us more susceptible to the survivorship bias, but also because schools often make us think we have competences we don't. The schools make us think we know how science works, but we are, thanks to the schools, not only ignorant, but also have a wrong conception about it. We are, for example, being taught linguistics and philology all the time in our Croatian language classes, and most of the people think that linguists and philologists are people who like to use some confusing arguments to discuss what is grammatical or stylistically right to say and what isn't. Nothing could be further from the truth, linguists and philologists are people whose job is to make testable theories about how languages work. But, if we think that linguists and philologists don't make testable theories, we see no reason to believe what they say about languages. And the same goes for most of the other sciences we are supposedly being taught at school. Some people advocate government policies like censorship to try to prevent pseudosciences from spreading. But, if you know anything about politics, you know that what politicians talk makes scientists dizzy. If the politicians were able to get rid of pseudosciences, they would first do that in politics. To the contrary, American politics is perhaps one of the main reasons the Internet is so full of pseudoscience. Namely, this isn't something people talk about a lot, but the American right-wing politicians (and sometimes also the left-wing politicians, but mostly the right-wing ones) hold a pressure on the media to support various conspiracy theories, because they are convinced that the science is some force of evil fighting against democracy and capitalism (and they are so fanatical that they do not see how absurd it is to do that if you actually care about democracy and capitalism). In the US, for example, it's completely normal to find an article in the newspapers claiming that the scientists are in a conspiracy hiding that evolution or anthropogenic global warming have been disproved. Why is it talked about so little? Well, partly because major social media platforms, such as YouTube, FaceBook and Twitter, routinely demonetize, remove from search results, or even ban those who are openly critical of the mainstream media (making it even harder to find out what the truth is). So, people are indoctrinated into that conspiracitory way of thinking, and apply it further. Everyone has to learn to recognize pseudoscience by themselves. What actually helps is knowing how people discover the pseudoscientific "knowledge".
They do what's called ad-hoc hypotheses. An excellent example of that is given by Carl Sagan. Suppose somebody claims they have a dragon in their garage. Of course, you don't believe them and ask them to show it to you. He says it's invisible. You ask him to spray some color on it to make it visible. He says that won't work because the dragon in his garage is incorporeal. So, you ask him to try to measure the temperature of the fire in its mouth. He says that won't work either and that the dragon's fire is heatless. I've studied many pseudosciences, and I can safely tell you, once you get deeper into some pseudoscience, that's the type of reasoning you engage in. You don't want to do an experiment because you know what the result would be. Scientists honestly try to falsify their hypotheses by experiments or observation. Pseudoscientists try to make up reasons why an experiment wouldn't work. You know, the horoscopes don't work because making one is error-prone. If you see such type of reasoning again and again in some field, you can be certain you are dealing with a pseudoscience.
Now, obviously, pseudoscientists are trying to hide that. They are making it look like their hypotheses aren't made up as excuses not to do an experiment, but that they actually logically follow from something. Biblical literalists claim their make-beliefs about the universe and atoms are based on the Bible. A little problem is: the Bible doesn't talk about those things. That's why the Biblical literalists can't agree on anything. They don't agree even whether the Earth is round of flat. Scientists, on the other hand, agree about those things. If people who study some field fail to agree on even the most basic things, you can be sure you are dealing with just another pseudoscience.
Now, the Internet culture gives us many clues how to recognize pseudoscience. Many of them will guide you right in the wrong direction. Many people, who want to hear both sides of some story, go to an Internet forum. The truth is, on Internet forums, you are happy to hear even one side of the story. If 100% of what's written by those who support pseudoscience is nonsense, and 90% of what's written by those who support science is nonsense, you are going to have a hard time trying to pick the right side. Many people say pseudoscience can be recognized by the authors expressing uncertainty. In fact, exactly the opposite is true. Pseudoscientists usually claim they are certain about what they claim, and scientists will often admit their uncertainty. The other thing we are often told is that pseudoscience can be recognized by searching for reliable sources. Well, let me say something heretical:
Wikipedia is the most reliable source of information on the Internet!
Wikipedia usually lets you hear both sides of the story about some issue and lets you make an informed oppinion. Other sources of information rarely do that. When Wikipedia is unreliable about something, then other tertiary sources of information (that is, those written by people who don't have very specialized knowledge of things they are writing about, such as encyclopedias or textbooks) are just as likely, if not even more likely, to be wrong about it. Common knowledge is not reliable source of information. There are many things that are believed by almost everybody, yet are false. For instance, almost everybody thinks that rabbits like to eat carrots, while carrots are actually quite dangerous for rabbits. Almost everybody thinks that mice like to eat cheese, when, in fact, cheese is lethal for mice even in relatively low amounts. It's hard to estimate how much of the common knowledge is actually false, but it could easily be most of it. And pseudoscientists use that to convince people. A good example of that could be the anti-nuclear movement using the well-known "fact" that Fukushima nuclear disaster killing many people by radiation. In reality, the number of people who have died because of the radiation is in the low tens, if not less than ten. The poorly organized evacuation is responsible for far more deaths than radiation is. But who would have thought such a well-known thing from recent history is completely false? Similarly, people who advocate meat-based diets (and other low-carbohydrate diets) often appeal to the "fact" that Inuits have very low rates of atherosclerosis, when they, in reality, have higher rates of atherosclerosis than the USA average (and the USA has above world-average rates of atherosclerosis).
So, how do pseudosciences come to be? Well, people come to bad ideas in a similar way they come to good ideas. You know when Francis Crick got his idea about the structure of DNA while he was on drugs? Pseudoscientists are usually way more educated in some field than an average person is. Many of them are engineers or physicians (and those are people who seem to have tendency to think they are somehow qualified to talk about things they don't know much about, especially the scientific method). It's not important who says something or how he came to some idea, what's important is that they understand the scientific method and honestly try to follow it. Pseudoscientists don't follow the scientific method because they don't realize why it's important, and that it would eradicate the errors they make. The errors that seem to repeat from one pseudoscience to another appear to be ignorance of statistics, ignorance of psychophysics (perception) and misunderstandings of the epistemology.
You know when pseudoscientists try to establish a relationship between some two languages, find even less apparent cognates than what's expected to be found by chance, and then claim it can't be due to chance, usually without doing any math whatsoever? Or when creationists claim it's extremely unlikely for a simple cell to emerge from non-living matter, without having any idea how to actually calculate that? Or, perhaps the most famous such example, when global warming deniers seem to think that any straight line drawn between two points on a graph is a trend-line? Or when anti-vaxxers claim that the studies that show that the number of vaccinated children who die in an outbreak of some disease outweighs the number of unvaccinated children who died somehow proves that vaccines don't work (despite of the number of vaccinated children being by order of magnitude greater than the number of unvaccinated children, and despite that nearly all unvaccinated children died, while only a small proportion of vaccinated children died)?
Or when the Flat-Earthers say things like that the fact that the horizon appears to rise as we climb proves that the Earth is flat? Or the conspiracy theorists claiming the photographic anomalies proving that the photographs are fake, even though they are exactly as some professional photographer would expect them to be? Or when the geocentrists claim that the sunrays not appearing to be parallel when the sun is in the zenith proves that the Sun isn't far away from the Earth?
Or when pseudoscientists use the Occam's razor as a justification for the personal incredulity fallacy?
I don't know about you, but I find the arguments based on perception most interesting. People uneducated about those things tend to have wild misconceptions about perception (like the emission theory of vision, see here), yet they tend to be rather confident their understanding of the perception is close to right, and that they are able to evaluate the claims made by pseudoscientists.
The most common type of pseudoscience on the Internet are conspiracy theories. It's basically when you go to an Internet forum or a blog and start yelling: "Everyone is a liar. They are poisoning us! They are brainwashing us! The fuels of jet airplanes contain poisons! The Earth is flat! The Moon doesn't exist! Wake up, people!" Look, the largest conspiracy theory ever proven to be true was the Operation Snow White, involving, according to some sources, around 5000 people with a clear motive: to hide the illegal things their religious leader had done. Compare that to the popular conspiracy theories. If the Moon landing was faked, it would take hundreds of thousands of people to be in a conspiracy. Namely, they left some mirrors on the Moon that are detectible by lasers and hundreds of thousands of people have detected them. As for the 9/11 conspiracy theories, look, there have been so many both governmental and non-governmental investigations of it, and it would certainly take at least thousands of people to be in a conspiracy. And if you look at the actual claims made by those conspiracy theorists, like the videos of the crash being obviously fake, it would take millions of people to be in a conspiracy. The claims such as those that the Croatian general Mile Dedaković made the Vukovar Hospital Massacre happen by colluding with Željko Ražnatović and secretly giving the weapons the president Franjo Tuđman sent him during the Battle of Vukovar to some Serbian paramilitary are about as absurd, but you need to have some specialized knowledge to realize why. Conspiracy theories put people in some imaginary world in which, well, science is some force of evil, nothing can be learned and nothing makes sense (Almost a definition of a conspiracy theory is the denial of the simple truth that a single whistle-blower would be enough for a large plot to fall apart.). And in which mainstream thought seems like either laziness or even as an attack on the conspiracy theorists (For example, when Caesar said he would like to hear the other side of the story of the Catiline conspiracy, Cicero, and many others, accused him of being a part of the conspiracy). That's why it's so hard to fight them.
Just before discrediting someone as a conspiracy theorist, make sure he really is one. For example, those who are skeptical of the policies made to address global warming are often grouped together with the global warming deniers, even though equating the two doesn't make any sense. I think the most dangerous pseudoscientists are not conspiracy theorists, who by definition know the science is not on their side, but people who have convinced themselves and others that the science is on their side. You know, like those who think global warming proves government stealing money is somehow good for the environment, or the people who supported Lysenkoism. They tend to be way more persuasive and it is usually harder to convince them they are wrong.
On the left, I've linked to a parody of the conspiracy theorists where I show how easy it is to find pseudoscientific and pseudophilosophical arguments for any position you can think of, no matter how absurd.
I hope I've helped some people deal with pseudosciences on the Internet, not spending substantial amounts of time researching some field before realizing it's pseudoscience, what has happened to me many times.

UPDATE on 22/09/2019: I've just posted a YouTube video about pseudoscience in American politics. If you can't open it, you can perhaps try to open a low-quality MP4 file hosted on this server (it can be opened on almost any platform using VLC Media Player). Failing to do even that, you can probably download the MP3 audio.

UPDATE on 08/03/2020: I've just posted a YouTube video criticising climate change denialism, you can see it here. If you can't open it, try this.

UPDATE on 19/04/2020: Here is the Croatian version of my parody of the conspiracy theorists. If you can't open it, try the DOC, DOCX and ODT.