The worst I've had happened about three years ago. I had the orginal DS and I had loaned it to a friend of mine for the weekend. He gave it back to me right before school started so I didn't have to time to look at it before class. As luck would have it, that day was one of like three days we didn't do anything in band so I decided to play some games. When I took out my DS I found a giant crack on the top screen and a black blob thing(coming from the crack). It still works fine and I could play GBA games on the lower screen and everything. I ended up giving it to said friend when I couldn't sell it and I bought a DS lite. I'm still kind of ticked I didn't check it when I got it back, even though I figure it probably cracked in one of our backpacks.

Moral of the story: Check you stuff when you get it back from friends. Just in case.