unknown unknowns or known unknowns or known knowns
"Reports that say something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know,"
"We also know there are known unknowns; that is to say we know there are some things we do not know.
But there are also unknown unknowns -- the ones we don't know we don't know."
--Don Rumsfeld--