Unkown unknowns

"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."
-Donald Rumsfeld

This classic example of linguistic dexterity seems to be very appropriate to some work I'm doing at the moment.