The Athenian philosopher Plato (428/7 to 348/7 BC) created the island of Atlantis as a fiction. Sadly, this has not stopped people from trying to find it.
The relationship between Achilles and Patroclus is sometimes portrayed as one of cousins, most notably in movies based on the Iliad, but their relationship was far more complex.
It is often repeated that the Spartans practiced an institutionalised form of eugenics, part of which included the disposing of “imperfect” babies . Modern archaeology, and historical studies, have brought this story under serious scrutiny.
It is a commonplace to believe that people in the past held that the Earth was flat. But already in the distant past, people realized that our world is spherical.