It still irks me that people mocked Rumsfeld's explanation of unknown unknowns. The guy was probably a terrible excuse for a human being, but he wasn't stupid, and that quote was a pretty good explanation for why we shouldn't be overconfident in our knowledge:
"There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know."
"There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know."