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Experience underflow glitch
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Major glitches of the Pokémon series

Arbitrary code execution (template, remote, cart-swap, unintended ROM code execution) | 2x2 block encounter glitches (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)) | 99 item stack glitch | Bad clone glitch (????? party overloading (Type 0xD0 move glitch, ????? map corruption, | Celebi trick, Celebi Egg trick, Shiny Celebi trick, glitch move map corruption, overloaded party map corruption, Glitch Unown (Glitch Unown map corruption) Duplicate key items glitch (infinite items and item creation, expanded Balls pocket (TM/HMs outside of the TM/HM pocket, Glitch Pokédex categories)) | Berry glitch | Bug-Catching Contest data copy glitch (Japan only) | Cloning | Ditto DV manipulation | Elite Four door glitch (Japan only) | Expanded party encounter table manipulation | Generation IV item cloning glitch (Japan only) | Glitch City (Safari Zone exit glitch, RAM manipulation) | Glitch meta-map script activation (Generation I) | Infinite Master Balls (Colosseum) | Large storage box byte shift glitch (storage box remaining HP glitch, maximum stat glitch) | Item duplication glitch | Item stack duplication glitch (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) | Mimic glitch (Japan only) | Out of bounds Glitch City (Generation II) (Slowpoke Well out of bounds corruption (French version)) Lumiose City save glitch | Pikachu off-screen glitch (Trainer corruption glitch) | Pokémon merge glitch | Pomeg glitch (Pomeg data corruption glitch) | Roaming Pokémon encounter glitch | (Mimic) Transform Rage glitch | Select glitches (Japan only) (Closed menu: Dokokashira door glitch (international), Fossil conversion glitch (international), Second type glitch, Skip to Level 100 glitch, Trainer mutation glitch, walk through walls (international) Special menu: Lift glitch, Badge describer glitch) | Sketch glitch | SRAM glitch (Generation I) (255 Pokémon glitch, send party Pokémon to a new game) | Surf down glitch (Grass/rock Surfing glitch (Spanish/Italian only) (English)) | Time Capsule exploit | SRAM glitches (Generation II) (Mailbox glitches, Mystery Gift item corruption, Trainer House glitches) | Tweaking | Trainer escape glitch (Death-warp, Mew Trick, Ditto Trick, Experience underflow glitch) | Buffer overflow techniques (Japanese unterminated name glitch item instant encounter glitch, LOL glitch, Rival LOL glitch, Instant LOL glitch, RAM LOL glitch, oobLG, blockoobLG, Instant encounter infinite chain glitch (LGFly)), Super Glitch (Generation I) (party remaining HP glitch), Super Glitch (Generation III), Text pointer manipulation mart buffer overflow glitch, CoolTrainer♀-type move, Double distort CoolTrainer♀ corruption, Yami Shop glitch) | Walk through walls (ledge method, museum guy method, Rival's effect, Select glitch method (international), Brock Through Walls) | ZZAZZ glitch (party Pokémon box data shift glitch)

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Gen III 00FC-0114.png
Reason given: Check underflow techniques for other experience groups, including glitch experience groups
A level 1 Mew with 'negative' experience (16777162) or -54.

PRAMA Initiative a également une page sur Experience underflow glitch.
Bulbapedia also has an article about Experience underflow glitch.

The experience underflow glitch refers to a 'glitch' in Pokémon Red, Blue, Yellow and Pokémon Gold, Silver and Pokémon Crystal where the result of the calculation of a Pokémon's experience was a negative number.

It also refers to a common glitch technique in Pokémon Red, Blue, Yellow where a level 1 'medium-slow' experience type Pokémon is forced to jump to level 100.

Underflow with medium-slow growth Pokémon

The medium slow experience type is used for all valid Pokémon part of 3-stage evolutionary line excluding the Butterfree, Beedrill, and Dragonite line. Mew is also part of the medium-slow experience type.

The experience calculation for this experience type is as such:
03 Medium-Slow.png , i.e. 1.2L3 - 15L2 + 100L - 140

For level 0 and level 1 Pokémon the result is a negative number; i.e. -140 for level 0 and -53.8 for level 1 (which is rounded to -54).

However, this is interpreted by the game as an extremely high positive number (16777076 at level 0 and 16777162 at level 1), and the maximum experience is 16777215 (FF FF FF) before the total experience 'wraps around' back to 0.

This means that if a level 0 Pokémon gains less than 140 experience or if a level 1 Pokémon gains less than 54 experience it will jump to level 100 if it is stable. If it gains more than the relevant amount it the experience will be a positive number so the Pokémon will jump to a normal level.

Despite the experience being for an extremely high level, level 100 is the cap. However, note that the level cap may vary for unstable hybrid Pokémon, for example the cap for a Mew/Chansey hybrid with Chansey as the recipient is level 109.

Obtaining level 1 or 0 medium-slow experience type Pokémon

Obtaining level 1 medium-slow growth Pokémon is possible with Special stat encounter Trainer escape glitches, if the enemy Pokémon's attack stage is 1 (i.e. if six Growls have been used).

Obtaining level 0 medium-slow growth Pokémon is possible with a Super Glitch item in Pokémon Red, Green, and Pokémon Blue (Japanese) and Pokémon Yellow (Japanese). It is also possible with the text pointer item ball manipulation.

Example ways to get a level 0 Pokémon indirectly:

A medium-slow experience type Pokémon can be obtained at over level 100 using the old man trick and then leveled up to level 0 (by trying to level it up to 'level 256' with Rare Candies).

In Yellow, a Pokémon over level 100 can be hybridized with a medium-slow growth Pokémon that can evolve using the Pokémon merge glitch, and then the Pokémon can be evolved with a Rare Candy.

Withdrawal lockup

Any Pokémon with a negative experience value that is stored in the PC will immediately become "stuck," attempting to withdraw this Pokémon will cause a lockup. Here is a discussion from the forums,6588.0.html on glitch Pokémon experience types. Check under RB medium-slow families and Y medium-slow families as they will cause this lockup.

Daycare lockup

Similar to the withdrawal lockup, any Pokémon with a negative experience value given to the Daycare man, and taken out before the experience becomes positive, will cause a lockup.

See also