Where are the unknown unknowns?

Reports that say that 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. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

Quote from then United States Secretary of Defense Donald Rumsfeld, in answer to a question at a U.S. Department of Defense (DoD) news briefing on February 12, 2002.

I love this quote, because in software testing, we are always hunting the "unknown unknowns," that bug we believe exists but haven't caught up to yet, that new technique for a different kind of test we haven't tried before, and the hunt for the pernicious but tiny flaws that grow over time into horrible ugly nastiness.