The only one I can think of at the moment is, "people though the world was flat in Columbus's day," simply because it's erroneous. Just about every educated person in Columbus's time knew that the Earth was round.
True. The reason Columbus had problems getting anyone to sponsor him was that the natural philosophers of the day largely thought the distance between Europe and China was far greater than Columbus supposed it was. They considered it impossible for a sailing ship to carry sufficient provision to make the journey there and back, and they were right. The distance was too far to make it to China. What no one apparently considered was there might be unknown lands between Europe and China.
