Speaking of office pranks, last summer I was working as a summer student at an office downtown. There was this guy who rode his bike to work everyday, so he left his dress pants and slacks in his office to change into once he got to work. One of my female coworkers thought it would be funny to mess with his mind, so every friday she would take his pants home with her for the weekend and hem them a half inch shorter. A few weeks later and he had no clue what was going on. He kept commenting on how he must be getting taller because he gets these pants dry cleaned, so they can't be shrinking.
We let this guy walk around with flood pants for a week or two before we finally let him in on the joke. Took a lot of time on my female coworkers part, but it was well worth it.