Personal pet peeve: the idea that Columbus had this brilliant epiphany that the world just might be round. (Linked to that because the April 26 question mentioned that belief.)
When I was a kid I had some kind of National Geographic guide to the solar system. Interspersed with all artists' conceptions of life on Jupiter and the constellations fleshed out into the mythical characters on which they're based, there was quite a bit of science fact and history too. One of which was the revelation that not only did people have the shape of the world figured out by around 300 BC, but an Alexandrine librarian had made a good estimate of the size.
Yet as far as I can tell, nobody knows this simple and established fact. (And if you think about it, it's almost common sense: convincing a monarch that the size of the Earth is overestimated is one thing, but who would sponsor a guy crazy enough to say the world is a completely different shape?) Of course, the fact that people don't talk about this is mostly if not entirely because it's not as interesting a story as "Columbus showed them all the world was round!" For all I know the fact is central to classes on the history of mathematics, the Hellenistic era of Greece and other subjects I never formally studied, and it's not the default belief only because it's not as easy to use as an expression.
I don't even know why this annoys me. My pedantic side showing through, I guess.