Pokémon cloning (Generation I)

Requirements

• Two cartridges that are both Pokémon Red, Blue, or Yellow Version.
• The Pokémon to be cloned.
• A Pokémon to be lost during the process (not on same game as the one to be cloned).

Procedure

1. Link the two games and go to a Pokémon Center.
2. Go to the Cable Club and enter the Trade Center.
3. On one game, trade the Pokémon to be cloned, and on the other, trade the one to be deleted.
4. On the game originally having the Pokémon to be cloned, reset the game without saving. On the game where that Pokémon now resides, save.

Result

The Pokémon to be cloned should now exist on both games, and the other Pokémon should be overwritten by one of the clones.

How it works

Simply because the game was not saved after the later deleted Pokémon was obtained, the save data still says that the player has the cloned Pokémon. On the other game, it was saved with the cloned Pokémon on it, so then both games have the same Pokémon.