ClubSi had an awesome prank. About 1.5 years ago, they switched from an old version of their forum software to a new version. It was controversial (kind of like here) because everybody liked the old look much better.
So anyhow, the admin wrote a thread on March 31st saying "I ****** up" explaining how the database was corrupt, and his backup scripts had been backing up to a full harddrive and not getting them completely backed up, and his last complete backup was from Oct. 2001.
So everybody was like "Is it back!? The old forums?" people were pissed because of missing PMs etc with the new software, a bunch of banned members were back etc. But he put it back later that evening. It was funny, and some of the old posts they revived were hilarious.