The "known knowns" quote got made fun of a lot, but I think it's really good out of context:

"There are known knowns; there are things we know that we know. There are known unknowns; that is to say, there are things that we now know we don't know. But there are also unknown unknowns – there are things we do not know we don't know."

Also, every time I think of that I try to picture the elusive category of "unknown knowns" but I can't ever think of an example.