Fill someone's hair-dryer with baby powder.

 510

1  

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

 1061

1  

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

 610

0  

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

 613

1  

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

 557

3