This was a prank against the worst boss I ever had.
A horrible, horrible man who managed to even get complete strangers to hate him. He was very possesive of his office, to the point of insisting that our Admin Bloke moved from the office that had always been shared with the boss to the slightly smaller office next door, which had always been the Tech Bloke's office and share with him. Admin Bloke was obviously a bit put out at this and asked the rest of us to help him gain some small measure of revenge. My idea won the day.
The prank? We have an Office Cat, being a cat he quite regularly brought in mice to kill. For the next few months we'd get the killed mice off him on nightshifts, and hide them in the boss's office, usually under his foot-rest. Then turn the heating up full and relock the door. An hour before the end of shift we'd turn the heating back down again. By the end of the second week the room stank. The boss would call us in to ask if we could smell anything. Cue straight faces and negative replies. The cleaners were in on the joke, so made no effort to help him. We got the Air Trafficers in on it, and he started to call them in to ask too.
To the day he left he never worked out where the smell was coming from.
It turned out that one of my colleagues was also pursuing a personal prank against him too. The boss had a catering sized tin of Cadbury's Drinking Chocolate that he kept in his office so as not to have to share it with us plebs. Except that the level of drinking chocolate in the tin never seemed to go down that much. This was because my mate Pete had been secretly adding ground up dried flies to it for about a year.
( stuj (^(^;;^)^) GO TEAM SPIDERS!(^(^;;^)^), Fri 14 Dec 2007, 1:23, 2 replies)

Where did he get ground up dried flies?

( Bob Todd whee, Fri 14 Dec 2007, 14:10, closed)

Indeed.
Taking the time to catch, dry, and grind up flies?

When things like laxative powder or LSD are so much more easily gathered?
(A Wooden Leg Named Smith, Sat 15 Dec 2007, 18:28, closed)