The myth that Columbus proved that the world was round is not something that I encountered in my education in Sri Lanka. It seems to be a largely American creation, likely for all the reasons that cartomancer and jkrideau list. My first experience with hearing it was when one of the undergraduates in my class casually inserted it as an element in the argument he was making about something else, if it was the most obvious thing in the world. I stepped in to question him and was astounded in the ensuing discussion to find that quite a few members of the class believed the same thing. They said that they had learned it in elementary school.
I suspect that many such myths are learned early in life. Here’s an amusing story from when my daughter was in third grade. She came home from school one day and told me excitedly how the teacher had explained how white light was made up of different colors. The teacher had also told her that the great scientist who discovered this was Roy G. Biv! To say I was surprised is putting it mildly. I tried to gently correct her about who the scientist was without seeming to disparage her teacher, but my daughter was skeptical. Who was she more likely to believe: her teacher, a fount of authoritative knowledge, or her dopey old father? (Incidentally, in Sri Lanka Roy G. Biv never appeared. We learned the order of the colors by saying ‘vibgyor’ pronounced phonetically so that it sounded vaguely Hungarian.)
Teachers in elementary schools are not scientists or historians of science and so are likely to pass on erroneous folklore as fact though, as my forthcoming book will point out, even professional scientists are not immune from that failing. As Richard Feyman said after outlining the origins of quantum electrodynamics:
[W]hat I have just outlined is what I call a “physicist’s history of physics,” which is never correct. What I am telling you is a sort of conventionalized myth-story that the physicists tell to their students, and those students tell to their students, and is not necessarily related to the actual historical development which I do not really know! (QED: The Strange Theory of Light and Matter, p. 6)
But not all scientists are as aware as Feynman that they are merely propagating myths and not the full story. As we well know, if you repeat a ‘myth-story’ enough times and it gets passed on from generation to generation, it can acquire the status of a fact.