Take a squirt gun into the rest room stall next to someone and shoot little drops over the wall every couple of seconds while pretending to pee.
 774
0  

Put tape over the optical sensor of someone's mouse.
 867
1  

Install the Blue Screen of Death screen-saver on someone's computer.
 833
3  

Write a message on an upside-down paper cup that alludes to something horrible being trapped under it. Leave it on a coworker's desk or in a conference room.
 1484
1  

Paint a bar of soap completely with clear nail polish so it won't suds up.
 757
1