I let my brother borrow Fallout 3 maybe a week or two back, and after about three days he handed it back to me damaged beyond repair. He feigned innocence, claiming "the Xbox ate it." It turns out he likes to use his Xbox as a footrest and the majority of his games are nonfunctional. :/
He showed me his copy of Portal 2 and it literally looked like broken glass. I thought it would fall apart if I picked it up.
I'm yet to break a disc or cartridge of any kind, personally.