[Reaper]

My favourite office prank usually ends up being a finisher when people use it.

Get to work about 2 hours early one day. Find the target's chair and remove the seat from the base. Fill the center post of the chair with frozen raw shrimp (make sure they are frozen when you put them in). Reattach the chair seat to the base and wait for the hilarity to ensue. In a few days there will be a smell in their cubicle that they cannot locate. In a week there will be a smell that makes people turn their nose something fierce. In two weeks there will be a smell in their cubicle that powerfully envokes the human gag reflex when detected. The person will try anything to get the smell out of their cubicle including a variety of air fresheners. Eventually the smell will be obviously coming from their chair as the decomposition of the shrimp will make the smell so concentrated that it will basically leave a stink trail whenever the chair moves.

Two things about this prank: either know your victim well enough that you know they won't have a nervous breakdown or snap and kill you if they find out you are the culprit OR pull this prank the day before you give your two week notice on someone you hate.