Exactly What It Says on the Tin: You have action A, which will have consequences. These consequences will ensue. This is either the natural outcome of an action or an unexpected one. In both cases, it'll probably be played for laugh.
Coitus Ensues: You expected chastity. The plot comes to a screeching halt as sex happens.
Hilarity Ensues: Someone does something that sets off the motions of making you laugh.
Humanity Ensues: Regardless of your expectations, a character gains a human form.
