I have one copy of Mega Man 2 that works worse than another. The "bad" copy is the one I grew up with, and it's more likely to garble the graphics while playing. I've always wondered if it's a result of the ROM chips degrading or the connectors, but the connectors are pretty clean. At any rate, it's always funny to see the CHRROM calls get offset and all the text get cyphered. Then, the intro text often contains strong profanity. Considering how hard it was to get games to boot properly back then, I'm surprised there was never a controversy over kids seeing profanity in damaged games.

Theoretically, could data held on ROM chips be damaged by electrical spikes or overvoltage?