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The Johto guard glitch is a glitch in Gold, Silver, and Crystal which allows players to trade a Generation II Pokémon into a Generation I game as an equivalent glitch Pokémon or Generation I Pokémon by using a ????? (FF) (index number 255).
To use the glitch, the player will need to obtain a Generation II game, a Generation I game, and a Game Link Cable.
In the game, the player will need a ????? (FF) (index number 255). There are multiple ways to obtain a ????? with an index of 255. The Celebi Egg glitch can be used to get any version of ?????. To do this, the Celebi Egg glitch should be done with a Pokémon that has only two moves. This will produce a Egg that will hatch into a 'bad clone' of ????? (00), likely ????? (00) (index number 000) though there are ways to obtain a ????? (FC) (index number 252) or ????? (FE) (index number 254). If the player has a ????? (00), the player must deposit the 'bad clone' into the Pokémon Day Care on Route 34. Should everything go correctly, the 'bad clone' will become a ????? (FF). If the Egg hatched into ????? (FC) or ????? (FE), the player will need to train them to level 3 or 18, respectively, to have them evolve into ????? (00) before depositing the 'bad clone' into the Day Care. A second, though more random, method is to use the Pokémon Cloning Glitch to create a 'bad clone' on the file. The 'bad clone' must then be taken to the Pokémon Day Care on Route 34 to attempt to obtain a ????? (00) 'bad clone' after withdrawing it. If the withdrawn Pokémon is a ????? (00), it can be deposited back into the Day Care and withdrawn again to 'revert' it back into a ????? (FF).
Hiding Generation II Pokémon
This glitch takes advantage of ????? (FF)'s 'cloaking' abilities. If the ????? (FF) is switched above a Generation II Pokémon, it is possible to bring Generation II Pokémon into the Time Capsule, as long as each Generation II Pokémon in the party is hidden below the ?????. This is because Pokémon below ????? (FF) are temporarily regarded as being of index number 000 and ????? (00), index number 000, is not considered as a Generation II Pokémon by the game. In addition, ????? (FF) is regarded as index number 255, which becomes the "Cancel" function in the menu. As such, Pokémon below the ????? (FF) will be regarded as an 'M (00) in Red and Blue, or a 3TrainerPoké $ in Pokémon Yellow.
However, the game will still block the Pokémon if it has any moves which originated from Generation II even if ????? (FF) is above it, so any moves which originated from Generation II must still be removed by the Move Deleter if they are known by a Pokémon to be hidden by the ????? (FF).
The glitch is useful for obtaining glitch Pokémon in Generation I games, and can be used to obtain almost any glitch Pokémon in Generation I, including glitch Pokémon previously unobtainable via the Ditto glitch and old man glitch because of a procedure where an equivalent Trainer class is fought if glitch Pokémon with index numbers of 200 and above are battled in the wild.
Reverting 'M (00)/3TrainerPoké $ into an equivalent species
If ????? (FF) is switched back underneath the Generation II Pokémon after entering the Time Capsule, its original species is restored, meaning that Generation II Pokémon can be taken into the Time Capsule.
If the Pokémon is traded as an 'M (00) or 3TrainerPoké $, it is actually an unstable hybrid of 'M (00) or 3TrainerPoké $ and the actual glitch Pokémon, with the actual Pokémon as the recipient and the 'M (00) or 3TrainerPoké $ as the donor. To stabilize an unstable hybrid Pokémon, a Pokémon can be evolved (in the way the recipient would), traded to Pokémon Stadium 2, or deposited and withdrawn from Pokémon Day Care. A stabilized hybrid will be the species and level of the recipient Pokémon (or its evolved form if it was evolved), but have the stats, nickname, and moves of the donor. Once stabilized, the Pokémon can be traded to the Generation II games.
Since Generation II Pokémon do not exist in Generation I, the received Generation I Pokémon is determined by a conversion table for index numbers 1-250 which is hard-coded into the ROM of Gold, Silver, and Crystal. It is currently unknown what determines the equivalent Pokémon in Generation I for a Generation II Pokémon with index numbers 000 and 251-255 because the equivalent Generation I Pokémon is not consistent on the species for these Pokémon.
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Before a method was found of obtaining a ????? (FF) without cheating, the idea of using a ????? to bypass the Time Capsule check for Generation II Pokémon was proposed on TRsRockin's forums, but was never fully investigated until Paco81's discovery of the Celebi Egg glitch on Glitch City Laboratories.