My old sales guy had a guy at the last company that he worked for that was a royal PITA, so on April Fools he unscrewed the mouthpiece of his old style telephone and dropped a cooked prawn in, put the mic back in and screwed the cap back on. It took about 4 days for the prawn to start rotting, and of course it was only the guy that could smell something really foul, but had no idea where it was coming from. After a week, the smell went away.
I pulled one over on my sales guy a few years later. We were both playing the 'new' pinball game that came with one of the Windows versions, so I called him at home and told him that I found out something very cool with the game: Depending on how you tilted the monitor, it would affect how fast the ball moved on the screen, as it changed the amount of gravity of the ball.
He spent the next 2 hours that night playing pinball, and reported back the next day that I was right. Of course, there's no correlation between the game and the monitor, and he was the type of guy that rarely fell for anything, so I felt really good about that one.