The Incompatible Food Triad. Are there three foods that don't taste good together but every pair of them does?
There are many ways to interpret this "going together" but an example solution would be three pizza toppings -- A, B, and C -- such that a pizza with A and B is good, and a pizza with A and C is good, and a pizza with B and C is good, but a pizza with A, B, and C is bad. Or you might find three different spices or other ingredients which do not go together in some recipe yet any pair of them is fine.
(via josh)