I guess "unknown knowns" are the counterpoint to "unknown unknowns" -- things it never occurred to you to consider, but didn't. Eg. "We completely failed to consider the possibility that the economy would mutate into a continent-sized piano-devouring shrimp, and it turned out we were right to ignore that."
We completely failed to consider the possibility that the economy would mutate into a continent-sized piano-devouring shrimp, and it turned out we were right to ignore that.
That's a survivor bias.
Rationality quotes time!
The usual rules: