faaaak!
that's nasty, heads rolling tackle right there.
it's a sad fact of optical medias.
well, anything really, thermal dynamics and entropy at it's finest.

...
all the diseased joints i infest, this subject occasionally crops up, and the best solution would be to:
1, dump the disk image and verify it.
2, and create a 1/1 backup for a spoofed console.
(cloneCD and the tosec/redump/trurip dat tables to check shit against. dumping methods can vary, not a science, but an art.)
my saturn is just stored away til it gets an ODE.
there's RHEA, but out of my resource range atm, and i'll be waiting for a usb/sata version, which will eventually...

[edit]
typo.