I intentionally held off starting this thread until after April Fool's day.
But, The best, (harmless) prank I ever pulled was the careful covering of the mouthpiece of a co worker's phone with cellophane tape. The man had a BOOMING voice to begin, but when people on the other end claimed they couldn't quite hear him, he would need to yell! I feel a little bad because this went on for about 4 days. He had an IT guy come to check his phone. The dumb ass tech said all fine, nothing wrong with the phone.