People who fail to do basic research before asking a question. If your question is “how do I make text green in HTML” and it ends up on Stack Overflow then you deserve that downvote because apparently you couldn't be arsed to search for an answer first.
One of things that annoys me about SO is when you can't find the answer, but the question gets closed for being a duplicate because no one could be bothered to check if the question really was a duplicate. People assume that question X is a duplicate because it is a simple question. However, you cannot actually get from the information in the question to the solution using google or SOs terrible and unorganized search. Usually to find the supposedly duplicated question you need to have additional information, or the "problem" is the same, but the context is different.
For instance, I once was confused with unexpected behavior of a function giving me incorrect solutions. It turned out that the problem was that I had forgotten to instantiate one of the variables in my unit test. This was marked as a duplicate question. I found this very odd since I searched for "[math function] gives incorrect output on [input]" or whatever it was. So I asked about it and the duplicate thread in question was a generic thread on undefined behavior, which would require I knew what was wrong with the function in the first place.
In another instance, I had an issue with TensorFlow throwing a particular error. I find the question online and another user had the same problem as I did. This was closed as a duplicate because another user had gotten the same error message and discovered he had forgotten to run the steps necessary to get TensorFlow to work on the GPU. The problem is that neither I nor the question I found had forgotten to do that. In this case, the context was different even though the question was nominally the same. When I started a new thread explaining why that thread did not produce a satisfactory solution to my problem it was closed. The reason? It was a duplicate of the inadequate thread.
Neither of these would be that much of a problem, except that the answers on SO are overwhelmingly rude and condescending with no mechanism for remediation. On the few occasions where I tried to point out that you shouldn't tell a beginner that a solution is "obvious," that they "should be smart enough to figure it out on your own," or that a comment like "I don't understand how you managed to become a professional developer when you can't even write a proper unit test" is no appropriate I was told that "it doesn't sound rude to me." To be fair, rude and condescending seems to be a trait that programmers in general share, much like other careers tend to attract a certain kind of person. But ultimately, the primary problem with SO is also a culture at Stack Overflow which asserts that there is nothing wrong with SO, that it is perfect, and that it should not be changed in any way. When a very large portion of your user base is giving the same handful of complaints over and over again, perhaps the problem isn't in those users' heads.