Sarcastic dice are always funny....somewhere around here, if the cats haven't moved it, I've got a bag of joke dice - like, a d5, a d36, that sort of thing. I've been meaning to bring 'em along when I DM to see how long it takes the players to realize that something's funny.

And yeah, I have...a lot of stories about things showing up in places that those things were certainly not at the previous day/month/whatever.