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.
Scott Adams, on being an idiot:
...I proudly include myself in the idiot category. Idiocy in the modern age isn’t an all-encompassing, twenty-four-hour situation for most people. It’s a condition that everybody slips into many times a day. Life is just too complicated to be smart all the time.
The other day I brought my pager to the repair center because it wouldn’t work after I changed the battery. The repairman took the pager out of my hand, flipped open the battery door, turned the battery around, and handed the now functional pager back to me in one well-
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.)