When I went to the London meetup, someone mentioned the "punching someone upward in the nose can send the nosebone into the brain and kill them" urban myth, and we all nodded except the one guy who actually bothered to think about it and said "I don't think that can be right, it doesn't make evolutionary sense" or something on those lines. I think, in my case at least, this was "just" a cached thought from childhood, but it was quite humbling how many of us got something so simple so wrong.

I used to believe that altruism was generally faked. This was based on my direct experience (and perhaps some mind projection fallacy), and an assumption that personalities were consistent over time, or perhaps situation - so probably the good old fundamental attribution error. And a default assumption that high schools couldn't really just be terrible, because no-one would allow that to happen. Why did I believe that? I think not appreciating how fallible memory is, and overestimating the engineering of the human reasoning apparatus. Evolution is always stranger than you think.

I used to not believe in quantum mechanics or general relativity, because they were terribly explained. I guess again I was assuming too much good faith on the part of educators. In retrospect if I'd just found a college textbook I'd've straightened myself out a lot sooner than I did. The popular science publishing industry still seems dysfunctional, but presumably there are incentives that I don't appreciate that keep it the way it is.