Disc Rot?
I run an online game/CD shop. More than 10,000 discs CD/DVD/LD/BD have passed through me. I test each one.
I find scratched discs for sure. And moldy discs in cases that also have moldy manuals.
But a pristine case with a “rotted” disc?
Twice. 2 times in the 15 years I have sold games.