common misconception

This was going to be mine. There wasn't an educated person in Europe at the time of Columbus that believed that the earth was flat, and anybody who was not educated enough would have been concerned with something less trivial (i.e., daily survival) than the shape of the earth.

It also gives insight into why the Portuguese declined to finance Columbus. Portugal had the best navigators and ships in Europe at that time, and they knew that the world was much larger than Columbus was calculating, and that a westward sea voyage to India would not be financially feasible.

Incidentally, they were 100% correct, it's only that they did not know that two continents replete with gold and natural resources existed between them and Asia.

image source