unknown unknown
In the words of Donald Rumsfeld, something that we don't know that we don't know. C'mon Donald.
Each year, we uncover a few more of these unknown unknowns.
Unknown unknown
These are things that you don't know that you don't know, as opposed to things that you know you don't know. Coined by Donlad Rumsfeld, former US Secretary of Defense.
Boss: Is there anything that will cause this system implementation to fail?
You: That's an unknown unknown!
You: That's an unknown unknown!