What are some instances in history where something that was accepted and pure and scientific fact turned out later in time to be grossly and obviously false?

by ArabOnGaydar
Celebreth

This question has been removed because it's an "in your era" or "throughout history" question, which are not appropriate for this subreddit. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, you may PM /u/caffarelli to have your question considered for an upcoming Tuesday Trivia thread.