For no particular reason, I was recently thinking about the subject of bogus history—historical "facts" that are very widely believed and flatly false. The example I started with is one of some current political interest, the belief that Herbert Hoover responded to the beginning of the Great Depression by cutting government expenditure. As I pointed out some time back in response to such a claim, it's the precise opposite of what really happened. By the end of his term, Hoover had increased federal expenditure by about 50% in nominal terms, 100% in real terms (i.e. allowing for the fall in prices), 200% measured as a share of national income (which, of course, had fallen). By that standard, he makes Obama and Bush look like skinflints.
For a second and unrelated example, consider the standard story of Columbus—that he heroically stood up for the scientific truth of a spherical earth against a flat earth orthodoxy, sailing west in defiance of warnings that he would fall off the edge.
That again is almost precisely backwards, since in that controversy Columbus was the one holding out against the (accurate) scientific knowledge of the day. A spherical earth had been the accepted scientific doctrine for well over a thousand years and the Greeks had produced a reasonably accurate estimate of its size. Combine that with what was known about the width of Asia—by the end of the 15th c. quite a lot of people had gotten to China and back—and it was possible to calculate about how far Columbus had to travel to reach "the indies" by sailing west. In fact, he barely made it to the Americas, the width of a continent and the Pacific ocean short of where he claimed to be going. His justification consisted of fudging both numbers—claiming the earth to be much smaller than it was, the width of Asia much longer. Details available from Admiral of the Ocean Sea by Samuel Eliot Morrison. I'm going by memory, but reasonably sure of my facts.
What are other such examples—historical beliefs widely held and demonstrably false? Medieval witch hunts might be a candidate, large scale persecution of witches having started long after the end of the Middle Ages and being based, I think, on beliefs that the medieval church considered heretical. And I gather that the Spanish Inquisition has an undeserved reputation in that context, that being concerned with the serious issue of secret Muslims and Jews it regarded witchcraft accusations as a distraction to be dealt with by applying serious standards of evidence. And, from one of my areas of interest, there is the myth that medieval food was overspiced to hide the taste of spoiled meat.
But can anyone here offer examples as clear cut as my first two?