Not all your DVDs that you may assume to be pressed are folks. Short runs may in fact be burnt instead. A lot of Open University discs are done this way, even some DVD videos and multimedia titles. I know this for an absolute fact as whilst working as a DVD author I've loaded the damn replication machines myself.
As for an actual pressed disc becoming corrupted with no apparent signs of damage, not completely impossible. Sometimes the disc can have oxygen trapped in the disc that degrades the aluminium over time. It's quite rare with modern pressing methods but can occur with scratches or edge separation between the layers (moisture carries the oxygen to the aluminium via the scratches or gaps). Very rarely the inner layer can also overhang like the filling in a sandwich squeezing out past the bread. This is easily seen as small silver flakes that rub off the discs edge, this is bad news for your laser as if any flakes get on the lens they reflect the laser all over the place. This is a faulty disc pure and simple, again, this is very rare with modern pressing.
Writing meaningful data on a pressed disc? hmmm..... that I doubt, I've never heard of it being done. Writing a file on the hard drive that appends a disc and gives the illusion of an altered disc whenever it is inserted however, that isn't so hard to achieve.
