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

 883

1  

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.

 787

0  

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

 851

3  

Hide all of the desktop icons on someone's computer and replace the monitor's wallpaper with a screen-shot of their desktop.

 915

0  

Put a "Please Use Other Door" sign on the entrance to your office building if it only has one entrance.

 1502

1