I love in-game pranks. One of the first things our Guild Master did when we downed Patchwerk was to tell the entire raid to run through the frogger slimes, telling them to hit the slime to "get their buff". Sure enough, all the players who had never been to Naxxramas or read much about it proceeded straight through the gauntlet. The next thing we knew, there were corpses strewn all over the place and our GM proclaimed himself The Noob Killer.
Back in the days of vanilla WoW, one of our favorite things to do to new raiders was initiate them by telling them to go up to the 'window' behind Golemagg the Incinerator to take a peek at Ragnaros. Hilarity ensued and repair bills skyrocketed. I certainly enjoyed the brewfest brew trick on Sapphiron, although that might warrant some serious -DKP.
One of our officers' favorite tricks is to tell guildies to meet him in some major city, telling them he was going to give them something. When they finally get there, he pops a macro that strips him naked and casts Mirror Image, yelling, 'SEXYTIME!' No repair bills here, but seeing four naked trolls is certainly not worth the trip. How about you? What silly pranks have you played on guildies or friends (or even enemies)? What pranks have been played on you?