Duplicate key items glitch

Major glitches of the Pokémon series

No further extensions

Transform held item glitch (Generation IV, Japan only) | Mimic glitch (Generation IV, Japan only)

Buffer overflow techniques

Item stack duplication glitch (Generation I)

Generation I expanded items pack (Glitch Rocket HQ maps, Map FE (English and non-English European Yellow) | Map script pointer manipulation (arbitrary code execution | Map script pointer item ball manipulation) | Text pointer manipulation (arbitrary code execution | Item ball manipulation | Mart buffer overflow) | Trainerless instant encounter glitch

Bad clone glitch (Generation II)

Closed menu Select glitches (Japanese Red/Green)

Pomeg glitch (Generation III)

Pomeg data corruption glitch ("Glitzer Popping") | Charm glitch

Voiding (Generation IV)

Tweaking

Broken escalator glitch (Japan only) | Elite Four door glitch (Japan only)

2x2 block encounter glitches (Generation I)

Left-facing shore tile glitch (in-game trade shore encounter trick, Old man trick, Trade link up shore encounter trick, Fight Safari Zone Pokémon trick) | Viridian Forest no encounter grass tiles glitch

Glitch City

Safari Zone exit glitch | RAM manipulation | Out of bounds Glitch City (Generation II) | Slowpoke Well out of bounds corruption (French Gold/Silver/Crystal)

Large storage box byte shift glitch

Storage box remaining HP glitch | Generation I max stat trick

Pikachu off-screen glitch

Trainer corruption glitch

SRAM glitches

Trainer escape glitch

Walk through walls

Ledge method | Museum guy method | Rival's effect | Select glitch method (International Select glitch method), Brock Through Walls

Surf down glitch

Grass/rock Surfing glitch (Spanish/Italian only) (adaptions: Submerge glitch (international)) | 8 8 (0x7C) grass/rock surfing glitch (English Red/Blue))

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This Duplicate Key Items glitch is a glitch in Pokémon Gold, Silver and Crystal that allows byte shifting in the item pack, and using a more precise setup, the Balls pocket to contain more than the maximum amount of items.

Requirements

Two Pokémon Gold, Silver and Crystal games on two different Game Boys
A link cable
Access to the Cable Club on both games
Machine Parts and SecretPotion on both games

Performing the glitch

Give the Machine Parts to a Pokémon
Trade it to the other game
Take the Machine Parts off the traded Pokémon
Repeat with the SecretPotion (you can use the same Pokémon as before)
On the game with two SecretPotions and two Machine Parts
Put both Machine Parts in the last two slots of the Key Items pocket
Swap both items, which will turn into a single slot
Remove the Cancel item
Put both SecretPotions in the last two slots of the Key Items pocket
Swap these, and all bytes up to the Cancel in the Balls pocket will be shifted upwards

Expanded Balls pocket

If the item ID of the first ball in the Ball pocket will be written into the item counter, meaning that a Fast Ball (hex:A1) will give 161 items in the Balls pocket (which is far beyond the normal limit)

Arbitrary code execution in Pokémon Crystal


YouTube video by ChickasaurusGL

Requirements

Two Game Boys
A Pokémon Red and Blue or Pokémon Yellow game
A Pokémon Gold and Silver or Pokémon Crystal game
Another Pokémon Crystal game, that has a TM33 in the first PC item slot
A link cable
Access on all games to a Cable Club

Performing the glitch

Use 8F (Pokémon Red and Blue) or ws m (Pokémon Yellow) to obtain a Pokémon with the following nickname : (any character)x2, hex:C3, hex:F2, hex:D8, then whatever you want
Trade it to one of the Gen II games
On the other, perform the Expanded Balls pocket glitch