An obvious one: "Most people are very stupid." My own experience doesn't bear it out. My own impressions and models of the world run precisely the opposite direction. But enough people seem to believe it that I'll give it credence for that reason.
If I recall correctly, your education has taken place at particularly elite universities, which would tend to indicate that you've spent an atypically large amount of time interacting with people of very high intelligence. This may explain some of the disconnect between your impressions and those of other folks.
It's interesting to think about what this might mean, by thinking like a positivist.
Most people are stupid, relative to Old One, and most people are smart, relative to lichen. We need a standard reference point in order to assign a truth value.
But the only salient reference point for the speaker of that sentence is... the speaker of that sentence. Hence, the opinion reduces to:
"I am smarter than most people."
Now, that may be true about the speaker. But when everybody agrees about that... then we're living at Lake Wobegon, "where all the children are above average." Only now they're all below average (except for me, of course).
To jump off of what komponisto said, I had a similar reaction when I was at Yale also. Now that I'm a grad student at a less prestigious university I see a lot of dumb undergrads as well as dumb people in some other departments. It seems that the smartest kids at BU are as smart as the smartest Yalies. But there are a lot more dumb kids and the dumbest undergrads are much dumber than the dumbest Yalies.
What do you believe because others believe it, even though your own evidence and reasoning ("impressions") point the other way?
I don't know. If I did, I'd probably try to do something about it. But my subconscious mind seems to have prevented me from noticing examples. I don't doubt that they exist, lurking behind the blind spot of self-delusion. But I can't name a single one.
Feynman: "The first principle is that you must not fool yourself - and you are the easiest person to fool."
AI hard takeoff before 2100 might count as an example for me, although the issue is rather complex.
Also, I don't see any reason why things like quantum chromodynamics (or, for that matter, special relativity) shouldn't count, unless you're just trying to rule out a large class of predictable answers.
Ruling out a large class of predictable answers is part of it, but it's also arguably more a case of "impressions don't point anywhere" than "impressions point the other way". If challenged on that point I'd have to think about whether I could make it more precisely, but I was kind of hoping people would intuitively agree about the spirit of the question (which is why I didn't include the second paragraph at first).
Note that answers like "quantum chromodynamics" don't count, except in the unlikely case that you've seriously tried to do your own physics, and it suggested the mainstream was wrong, and that's what you would have believed if not for it being the mainstream.
ETA: added to main post as per Vladimir's suggestion.
What do you believe because others believe it, even though your own evidence and reasoning ("impressions") point the other way?
(Note that answers like "quantum chromodynamics" don't count, except in the unlikely case that you've seriously tried to do your own physics, and it suggested the mainstream was wrong, and that's what you would have believed if not for it being the mainstream.)