Here's some insider info. I used to be a test lead at a major game company making Xbox games. I know that M$ tried to crack down on this before I left, so I'm not sure how rampant it is anymore. But, I do know that if developers had a crash that they just couldn't track down close to RTM(release to manufacturing), they would mask the crash with a dirty disc error. So, even though it pops up a dirty disk msg, the disk might be perfectly fine and you have, in fact, found a crash in the game that was spackled to pass certification instead of being fixed.
Testing = Red-Headed Stepchild of game development