Rationality quotes time!
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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.
Information that was relevant and available but not considered at the time of decision making I'd consider unknown knowns.