Galoob v. Nintendo
The 1992 ruling that a cheat device does not create a derivative work of the game it alters.
The Game Genie, designed by Codemasters and sold in the US by toy company Galoob, sat between the NES cartridge and the console and overwrote individual values as the game read them: infinite lives, higher jumps. Nintendo argued that every such tweak produced an unauthorised derivative of the game, and in 1990 won an injunction that kept the Game Genie off shelves for about a year.
The Ninth Circuit sided with Galoob in 1992. A derivative work has to incorporate the original in some concrete or permanent form, and the Game Genie produced nothing that survived switching the console off. Altering a game for your own use at home was fair use besides. Nintendo ended up paying Galoob $15 million out of the bond it had posted for the injunction.