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 pair of rather large (and LOUD) firecrackers, and wired them to light bulb bases with nichrome wire such that applying power would set off the firecrackers. One of the devices was installed in the victim's room, the other in the perpetrator's.
That evening, the victim returned to his room, and turned on the light. There was a short delay as the fuse burned, and during that delay the victim (quite naturally) turned to see what was wrong with the light--BLAM! Well, the victim was (quite naturally) upset and flustered and resolved to do some- thing appropriately horrendous to the perpetrator's room. He went over to the perpetrator's room, his own counter-prank in hand, went in, and turned on the light. When nothing happened, his reflexes again betrayed him: he looked up at the light--BLAM!
Apparently, the victim never bothered to counter-stack the perpetrator.