turn persons speakers up really loud..change the start up sound to custom audio...load curse words or porno sounds

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At someones home, you may not like so much...take a dump in the back part of the toilet.stinky stinky.
For some fun confusion, change around some peoples printer cords, so that they print in different cubicles...hehehehe, usually takes a while for people to catch on.
While someon is looking away, or gone from their desk, if they have a coffee they bought or something to that affect, pluck hair from head, hold tight in one hand, curl using nails from other hand...instant pubes...drop into cofee...enjoy aftermath of most disgusted person you will ever see.
Place a sign on a door 'Wet Paint'..people walk through so awkwardly, and the ones who bump into it will do the funny squirm around looking for paint dance.lol.
Screw someones office chair to the floor..just far enough away from the desk to be really uncomfortable, and needing to be moved.
Hide a walkie talkie in someones office..and random points, either make wierd noises, or turn on a radio and broadcast it...mwuahahahahaha. If you do it right, where it's kinda quiet..people will be like 'WTF' all day, and ask others and look like nut jobs.