When I was young, we were told that our appendix had no useful purpose, it was a evolutionary leftover. In fact, Wikipedia articles still maintain this position:
Human vestigiality

Now, scientists think that they have found what the appendix does:
What is the function of the human appendix? Did it once have a purpose that has since been lost?
Scientists may have found appendix’s purpose

This raises an obeservation: at one time scientists were sure the appendix had no purpose, now they think it does. Clearly, this means the earlier statements were wrong. This means the scientists at the time were stating as fact things they did not actually know. Granted, scientific discovery is an on-going process, but there is a huge difference between "we don't know", and "it has no purpose".

This raises a question: what else have scientists claimed to know that has since been refuted, and what currently strongly held ideas will be refuted in the future?