Donald Rumsfeld famously identified three classes of knowledge: known knowns, known unknowns and unknown unknowns. The three knowledge classes map surprisingly well onto requirements knowledge but are all outside the scope of this panel. Our focus is on what Rumsfeld missed: unknown knowns. In RE terms, an unknown known is knowledge that a person (say, a customer) holds, but which they withhold (say, from a requirements analyst). They may withhold the knowledge deliberately for some perceived personal advantage. They may withhold it accidentally, perhaps not realizing the value of their knowledge. They may strive to share the knowledge but end up withholding it because they are unable to articulate it. Or it may be knowledge they don't even realize they hold. In each case, the unknown known fits at least one of several definitions that exist of tacit knowledge (TK).