A few of devianART's pranks.
Namely the "program reads your mind and draws what you're thinking of."
I still got a pretty cool result from the deviantART mura program, though
And of course, the infamous "i herd u liek mudkips" event. Oh, the Mudkips event...
I spent a few minutes thinking someone had actually hacked into the site and changed all the avatars for a laugh, until I noticed the date.