A character is insulted. Later, they get a flash of inspiration and think of the perfect comeback (which may or may not be actually witty, but they're very proud of it). So, of course, they try to arrange matters so that they get to use it--either setting up the original scenario in which they were insulted, or just using it later out of context. If the latter type appears late enough, it may qualify as a Brick Joke. Examples of Comeback Tomorrow include: