From the days of newspaper TV Guides that would write two sentence synopses (loglines) of a television show. Sometimes these summaries wouldn't sound comedic on the surface, so "hilarity ensues" was used to ensure the audience understood it was a premise for a comedy.

An ending that can be put onto the ends of stories and plans. A convenient phrase to use when your story or plan sounded really dumb. It usually doesn't make much sense.

Something funny is upcoming due to a build-up of circumstantial events, or a comedy of errors.

Sometimes involves rape.