[GolfPopper]

I had a corrupted save of Ultima III for the C64 on a disk. The game worked normally, but the world data had been corrupted into a bizarre mis-mash of all available terrain types and monsters. I kept that disk around and periodically loaded it just to fool around.

[anthk]

Pokémon Blue/Red/Yellow had a similar glitch.

[PlunderBunny]

I think you could deliberately insert the wrong disk at some point and get corrupted terrain that had an ‘infinite’ chest in it (or was that Última IV?)