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office pranks...
Here's a few fun ones we've pulled at my last job:
If the person who is first in the office has a cubicle, tape the door of their cube with clear, non-reflective packing tape from top to bottom. Set up a camera. This trick isn't foolproof, but often the person who comes in first and makes the coffee is either groggy or simply so comfortable in their environment they don't catch the makeshift door.
One year, we taped up someone's cube door and filled the entire cubicle with shredded paper from the top. From experience I would say use packing peanuts instead if you want to try this trick... easier cleanup!
And my all time favorite, the shrimp chair... this one takes serious dedication, take apart someone's office chair, stow a few frozen shrimp inside, and watch the fun begin. In a few days the smell will get awful, and finally they will figure out it's the chair... but this one is the gift that keeps giving, usually they will simply swap the chair with another unsuspecting coworker.