Celebi glitch (party method)

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))

(view, talk, edit)

The Celebi Trick is a glitch in Pokémon Gold, Silver, and Crystal. The glitch allows players to, through repeated Pokémon cloning, obtain any Pokémon -- including Celebi.

Requirements

The player must have beaten the Elite Four.
The player must have a box in Bill's PC that isn't full.
The player must have a Sneasel that knows Beat Up as its third move. Sneasel learns Beat Up at level 57.
The player must have a Pokémon that they don't care about stored in the PC. This Pokémon will be deleted during the glitch.

Procedure

Clone Sneasel correctly four times.
Clone Sneasel incorrectly to get a bad clone.
Put the original Sneasel in the first slot of your party, followed by the four good clones and the bad clone.
Store your bad clone in the Pokémon Daycare, and then immediately take it back.
Move the bad clone to the front of your party.
Access Bill's PC using the computer in the Daycare.
Choose "Move PKMN w/o mail", and move a Pokémon from the PC to the front of your party; for this example, we will use a Rattata. Note that this Pokémon will later be permanently deleted.
Turn off the PC and store your bad clone -- which should now be in the second slot of your party -- in the Daycare again. This time, do NOT take it back yet; instead, go back to the PC.
Deposit the soon-to-be-deleted Rattata and the four good clones, and leave the PC.
Go back to the Daycare man and take back the bad clone. Then, ask the man to raise the original Sneasel. At this point, the Sneasel's name will probably have changed to SEL.
Take the Sneasel back from the Daycare. It should have changed into a Celebi. It doesn't count toward your Pokedex, and its level may be abnormally high.
If you look at the PC box where you deposited Rattata and the four good clones, you will notice that the third and fourth clones' species (listed on the first page of stats) have changed, but their sprites have not -- they still look like Sneasels. When you withdraw these clones and put them in the Daycare, they will transform.

Credits

This glitch was discovered by Paco81. Matt12 helped translating and writing out a simplified version.

Notes

For some reason, in Crystal, if you look at the stats of the second clone in the PC box, the game might freeze or glitch.

This glitch can also be used to get any Pokémon in the game. The way this glitch works is the third move of the Sneasel we used was Beat Up. Beat up has a hexadecimal identifier of FB; Celebi shares the same identifier. So if you changed Beat Up to, say, Detect -- C5, in hex -- then the Pokémon that has the same hex identifier of C5 -- Umbreon -- would be the end result. It is the move, not the Pokémon, that matters; Sneasel is used because it is the only Pokémon that learns Beat Up.