List of cloning glitches
Pokémon cloning is a glitch that enables a player to make an exact clone of a Pokémon. This can be done in many different ways depending on the generation, but in some cases, like most glitches, can corrupt the saved data, forcing the player to start a new game.
Generation I
The cloning glitch in Generation I was discovered by accidental trades, where dirty connectors or damaged link cables where responsible of one of the players to receive a cloned Pokémon. This was due to the lack of capacity of the link cables to send and receive the data at the same time, so in some cases interrumping a trade resulted in cloning a Pokémon.
This glitch is very hazardous to the save file, because a trade needs to save the player's party in order to start the second part of the trade. But if performed wrong and the game has not finished saving, it will corrupt the saved data and force one of the players (or sometimes both) to start a new game.
Generation II
In Generation II the cloning glitch was not removed from the trades, although now it was not hazardous for the save file. There was also another way of cloning which involved depositing the Pokémon in the PC and then switching the box and shutting the power off while saving. This makes the game to save the changes inside the PC, but not outside. So, the player's party was unmodified and kept the Pokémon while the PC saved the Pokémon inside.
This also lets the player do a variety of massive tricks like cloning five Pokémon at the same time or get the three starter Pokémon by saving, starting a new game and avoiding saving until the player can deposit his or her Pokémon in the PC, and do the "save trick" again. Also a player could get a lot of valuable items, such as the Master Ball or PP Ups by making the Pokémon hold them and then cloning it.
Generation III
Due to the capability of the link cable to send and receive data at the same time, the auto cancelling of the trades if something gone wrong, the visual interface of the PC boxes and the data corruption-protection of Generation III, this glitch was supposedly fixed. But it was later re-discovered in Pokémon Emerald, where the Link-Battle mode of the Battle Tower saved only the party of the player, even if the player chooses "no" when the game asks to. This was due to the game's need to save massive data and check connections.
Generation IV
In Generation IV, much of the data structure remained much the same as Generation III, Pokémon cloning is mostly impossible as there is no use for link cables anymore. But Pokémon cloning in this generation is similar to the glitch in Generation II, but this time depositing a Pokémon for trade in the GTS rather than in PC boxes.
