Pokémon Red/Green/Blue dokokashira door glitch

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The dokokashira door glitch (known in Japanese as どこかしらドアバグ) is a Select glitch that exists in Japanese Pokémon Red/Green and Japanese Pokémon Blue.
This glitch allows the player to warp to various places - including the Hall of Fame - very early in the game. In fact, the glitch becomes impossible to perform later on in the game.

0:00 Intro
0:40 New game
1:33 Select glitch
1:44 Swap Pokémon
2:11 Dokokashira door glitch
2:26 Warping to Hall of Fame
3:07 Removing the glitch
3:20 Warping to Route 7
3:38 To the Celadon Mansion
4:04 Getting Eevee x25
4:20 Ending

There's an end party terminator in memory at the address D123 + its value + number of Pokémons. When you only have one Pokémon, this address is D123 + 1 + 1 = D125.
Swapping your only Pokémon with Oak's Parcel will move this terminator at the top of the party. The game gets confused as it can't find a terminator anymore so it extrapolates what it finds after the 6th Pokémon slot and assume they're all Pokémons.
Because these "Pokémons" aren't valid Pokémons, they may have weird attributes, including being poisoned. Thus, the game will behave as when a Pokémon is poisoned: it will decrease a value by 1 every 4 steps, thinking the value is the Pokémon's HP. It will decrease values until reaching an FF value (which, in this case, is supposed to be the terminator we moved), and because we're at the beginning of the game there aren't many FF values to stop it.
One of the value it will decrease is the one at the address D335. This stores the map that the player will warp to when entering a door. This allow to warp to different places, from different places, by decreasing the value every 4 steps.

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