Unquestionably my favorite was Dihydrogen MonOxide...

The favorite prank that I pulled though was when I worked for Bell Laboratories. I sent a memo around that stated:

"On 4/1/92 we will be clearing out all of the unused bits that are laying about in the Datakit®, RS-232, and LAN cabling throughout the facility. This will entail the injection of 150PSI compressed air into all these lines. As a result some of the bits and loose dust will be forced out of the lines and may be emitted with some great force from your workstations and terminals. If you want to prevent this please stop by the SAG office and pick up plastic bags which we will be issuing for this event. "

Mind you, with all the very intelligent people working there I figured that everyone would see through the gag and just laugh it off as the attempt at humor that it was. However, on April 1 when the air compressors were to allegedly do their thing there was a line going for quite a way up the hallway of people looking for their SAG issued plastic bags. Fortunately I had the foresight to bring a box of landscaper sized plastic bags with me to work that day. I ended up going to pick up more at the hardware store at lunch time. The topper for the day was the woman who had left work already who came running back in her workout clothes in a panic wanting to know if was too late to pick up her bag.