Well... When I was a younger lad, one of my favorite jokes to play on folks was to install a program called 'Drip'. This was an 'invisible' TSR that would act at random (but decreasing) time intervals, selecting a character from the screen-map and 'dripping' it down the screen until just before it contacted another character. Of course, this didn't hurt anything, but certainly caused some humorous reactions (e.g. "EEEK! My screen is melting!!").
A simple Simon Says style game for use on a Moterolla HC11 processor.