Time Capsule exploit (aka Johto guard glitch) (Generation II)

Read description for more information:

I've started a more in depth description of this glitch as an article on Bulbapedia, you can read the new article there.

The Johto guard glitch makes it possible to transfer second generation Pokémon from Gold and Silver into an equivalent glitch Pokémon. The species of Pokémon underneath the ????? become 'M (Red/Blue) or 3TrainerPoké (Yellow), but can be converted back into an equivalent glitch Pokémon if they are switched above the ????? again. Alternatively it can be taken into the Pokémon Day Care Center on Route 5, where it will revert back into the equivalent species.

The equivalent Pokémon for species 1-250 are hard-coded into the ROM of Pokémon Gold/Silver/Crystal. A list of equivalent Pokémon added by Koolboyman can be found on Bulbapedia's list of Pokémon by index number (Generation I) article. However, the equivalent species for index numbers 0, 251-255 are determined differently.

Steps:

To perform this glitch, a ????? (hex 0xFF) is required. A ????? hex 0x00 can be obtained by obtaining a bad clone in the bad clone glitch, then taking it into the Day Care and out again. After using move Pokémon w/o mail to bring a Pokémon to the top of the party when you already have 6 Pokémon to obtain 7 Pokémon, it can then be taken into the Day Care Center on Route 34 and back out again to become a ????? hex 0xFF. A link cable is needed for this glitch, as well as a first generation Pokémon game and a second generation Pokémon game.

The Gold/Silver/Crystal player needs to have access to the Time Capsule, by meeting Bill in Ecruteak City and waiting at least one day. Access to the Cable Club is needed on the first Generation game.

Additionally, all moves exclusive to Gold/Silver/Crystal must be deleted because although ????? cloaks the Pokémon species below it, it does not hide their attacks.

1) Have the ????? at the top of the party and Second Generation Pokémon below it, with any G/S/C exclusive moves deleted.
2) Link a first Generation game to the Pokémon Cable Club, whilst at the same time entering the Time Capsule in the second generation game.
3) Swap the Generation II Pokémon above ????? (hex FF) to convert it to an equivalent species

Background:

This was a glitch proposed on TRsRockin as a theoretical method of obtaining any glitch Pokémon in the English Pokémon Red/Blue/Yellow, before a method was found of obtaining ????? (hex 0xFF).

After Paco81's discovery of the Celebi Trick and its submission to Glitch City Laboratories, a legitimate method was found of obtaining ????? (hex 0xFF), so the glitch was further investigated on Glitch City Laboratories forums. It was posted on the TRSRockin forums that a ????? allows the player to circumvent the game from restricting post-151 Pokémon into the Time Capsule if it was placed into the party above other Pokémon.

It was found on GCL that Pokémon below ????? would become an 'M/3trainerpoké when traded back to Pokémon Red and Blue and the glitch was presumed to be false and was mainly forgotten until recently.