Perpetrator Jokes

My favourite two campus practical jokes:
1. Arrange the light switches in a lavatory and an adjacent room so that
flipping one switch also flips the other. Consider the resultant scene
(visible from the outside just by watching the windows): the room's resident
retires for the evening, turning off his (or her) light. Some time later,
someone else decides to use the head, and turns on the light. After a few
minutes, the resident awakes, decides that someone is pulling a fast one,
and turns the light off... on the guy (or girl) who is by now seated on
the throne! If it works correctly, the two unfortunates end up standing
at their respective light switches, with the lights oscillating rapidly
(by which time the perpetrators are rolling on the ground outside laughing
their heads off).
2. This prank happened a number of years ago. Two guys had been pulling
practical jokes on each other for quite a while. Finally, one of them got
a more...