I honestly forget which key it was, but it got pressed a lot during programming (I have to do matlab for my gen eng requirements)- I think it was the multiply sign. anyway it just one day disconnected at one end, and fell off shortly after. I didn't abuse it or anything, and didn't hit it THAT often. I think it was just one of those things, and it wouldn't have been a big deal if I didn't have to fight them over it. They didn't even need to do labor, just send me a new keyboard to install.
My former roommate broke one of the keys on his laptop as well (except he dropped a book on it LOL). It was the backspace key.