Here's a prank submitted by J Reed:

Easiest prank ever! The keys on a key board tray pop off real easy. While your mark is out, swap the position of a couple common letters, i.e. "m" and "n", "j" and "h", etc. A hunt-and-peck typist will take forever to figure this one out, and you  have the joy of listening to him try and enter passwords and e-mails with no success! What fun!

Great office prank J!