"Frankenstein" on the NES, an old, really lousy Castlevania ripoff, actually had a Game Genie code that made you "almost invincible." You wouldn't take damage on hit, but if you picked up health powerups you would lose health. It'd also only last until this one point in the last level where you have to swing over a big long pit of acid. You weren't immune to the acid even though you were immune to all other damage, and for some reason the code would just WEAR OFF once you got across.
Bearing in mind this is a Game Genie, which is supposed to basically re-wire the game's own code to the player's twisted whims, I'd say this qualifies as the worst cheat code of all time.