Cheating on a pact
Consider a situation where two guys make a pact, that they will stop doing something that they normally enjoy doing. Now depending how this pact came about to be, one can't really know which one of them, or both, is actually joking about the whole thing.
So naturally, since they normally enjoy doing the thing that they agreed not to do, each will definitely enjoy cheating on the other guy. But the enjoyment takes on a different nature when one of them gets discovered ...
From now on the first guy, who was discovered, will have a guilt-complex about cheating anymore. Hey will continue cheating, since he can't resist it, but he won't really enjoy doing it. But the other guy will have a whale of a time - he gets to continue cheating, and in fact, his enjoyment is now enhanced by the other guy's guilt! All he has to do is be more careful about it, and continue harping on the fact that the first guy was discovered cheating!