Unknown unknowns
Unknown unknowns have been largely made famous due to a phrase from a response the United States Secretary of Defense, Donald Rumsfeld, gave to a question at a United States Department of Defense (DoD) news briefing on February 12, 2002 about the lack of evidence linking the government of Iraq with the supply of weapons of mass destruction to terrorist groups, and books by Nassim Taleb (The Black Swan: The Impact of the Highly Improbable by Nassim Taleb, Random House, 2007).
Note
Turkey paradox
Arguably, the unknown unknown is better explained by the turkey paradox. Suppose you have a family of turkeys playing in the backyard and enjoying protection and free food. Across the fence, there is another family of turkeys. This all works day after day, and month after month, until Thanksgiving comes—Thanksgiving Day is a national holiday celebrated in Canada and the United States, where it's customary to roast the turkeys in an oven. The turkeys are very likely to be harvested...