Hacking Netflix has a post about the possibility that some users may be switching damaged discs from their personal collections for Netflix copies:
How often does this happen to others? Is this an act of Netflix sending out alternate versions of what is supposed to be in their inventory, or are customers trying to rip off Netflix by returning their own full-screen and rated discs?
It’s interesting. I was actually tempted to do this sort of thing myself once with a Farscape disc that I thought might have been damaged. But then of course I realized something: that would be wrong.