There are known known, and unknown knowns. And known unknowns, and unknown unknowns.
Unknown knowns are relevant facts of which you are aware but whose relevance to the problem you're trying to solve has not occurred to you.
Known unknowns are things you know you don't know.
Unknown unknowns are things you don't know -- and you don't know you don't know them.
Known knowns are, of course, things you know you know. We should note
(a) Some of these you only think you know, but reflect "facts" that aren't true, and
(b) we should add a category of "ignored knowns" for things we know but prefer to ignore. For example, even the stupidest bankers knew that loaning money to people who can't afford to pay it back couldn't be a good overall strategy, but preferred to ignore this fact (or cynically tried to arrange it so it would be somebody else who would get stuck).
Unknown knowns are the things Rumsfield forgot, as outlined by Zizek, quoted by "Long Sunday" here:
"In February 2002, U.S. Defense Secretary Donald Rumsfeld engaged in a bit of amateur philosophizing about the relationship between the known and the unknown: "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."
For Rumsfeld, these "unknown unknowns" represent the greatest threats facing the United States. But Rumsfeld forgot to add the crucial fourth term: the unknown knowns, things we don't know that we know-which is precisely the Freudian unconscious, the "knowledge which doesn't know itself," as the French psychoanalyst Jacques Lacan used to say. In many ways, these unknown knowns, the disavowed beliefs and suppositions we are not even aware of adhering to, may pose an even greater threat." Zizek, Iraq's False Promises