I used to frequent a message board that would give snarky/silly errors when you caused an error or when the board (inevitably) broke down. It would tell users things like "AHA! We caught you being sneaky!" or "Does your Mom know that you're doing that?". Dumb little sentences, but at least it's more interesting than your typical message board error.
Make sure you check out insidethecbc.com, there's a lot of really interesting information that comes out on there.