From Bulbapedia, the community-driven Pokémon encyclopedia.
Glitches are anomalies in software programs, including video games. They can cause various problems ranging from the purely graphical to completely wiping entire chunks of saved data.
They are usually caused by problems with a game's code, or from a player doing things the programmers did not anticipate, therefore causing the game to react unexpectedly.
In Pokémon RPGs, there are many glitches, due to the complexity of Pokémon. Glitches are listed below (bold = most famous).
There are three Glitches (the first two are only available in the Red, Blue and Yellow versions; the third one is available in the Gold/Silver and Emerald versions, and the Hall of Fame one is very minor) that do not necessarily always create glitched Pokémon, but are themselves glitches that cause events that would not normally happen in the gameplay.
- Glitch City: a mess of pixels that changes depending on where the player is
- Mew glitch: allowing players to capture any Pokémon (most especially Mew) without modification, and allowing players to catch most of the glitched Pokémon listed below
- Hall of Fame Glitch: When Missingno. has been seen on the save file, the player's Hall Of Fame is badly corrupted with entirely different Pokémon (players can even see Mew in the Hall Of Fame, whether they've seen Mew or not) and very glitched characters and names.
Generation I - Generation IV:
- Pokémon Cloning: Variations of the Pokémon cloning trick exist in each Generation of the Pokémon games, each slightly different from one another. The most reliable verison is in Generation III. This Pokémon Cloning version allows players in the Gold/Silver versions to safely clone Pokémon, assuming they have at least somewhat decent timing.
- Sprite glitch: Glitches that mess up sprites have occured in all generations (mostly by cheating)
- POMEG Glitch: allows players to have a Pokemon at 65535 HP. Also allows players to battle with EGGs and ??????????.
- Man on roof of Cinnabar Gym: If one does not have the key to the Cinnabar Gym, and surfs on the east coast and returns to land directly infront of the gym, a man will appear on the roof of the gym.
- Prevented progress: If one evolves their starter Pokémon before they obtain their Pokédex from Prof. Oak, the game will assume that, since they have 2 Pokémon registered, that they already have a Pokédex, and will not allow them to proceed.
- Glitch dimension: Turns everything into strange, random colors and slows the ability to see the party screen and changes the "Poké" in Pokégear to random symbols
- Dratini Glitch: After the player speaks to the Dratini in Blackthorn City and uses the Coin Case, the Coin Case's display will change what the coin case says from "Coins: ..." to "Which move?he pp of" permanently unless the game is turned off.
Main article: Glitch Pokémon
, which accounts for the hexadecimal slot 253 in the Gold/Silver versions
A glitch Pokémon is a Pokémon that exists in a game due to a programming or level design oversight or for beta testing, and as it was not intentionally placed inside the game by Nintendo or Game Freak, it might be hazardous to saved data. They were originally supposed to be place-holders for unused Hex-adresses in the game. Glitch Pokémon are to be avoided due to the risks they may pose to a player's save file.
This is a chart of all 24 known glitch Pokémon that can be found in the wild in the Red, Blue and Yellow versions without using a cheating device:
The following glitched Pokémon can be found in the Gold/Silver versions, but require Gameshark to complete them. (Note: after capturing these Pokémon, the game's graphics have been known to screw up, and the player will find themselves in a Glitch City-like area.)
A glitch attack, or glitch move, is much like a regular attack except the results are often undesirable. These attacks are often known to be called TM attacks (TM01-TM55 and HM01 to HM05), and are also known to have glitch types (COOLTRAINER-type or 999-type)
Like a glitch Pokémon, glitch attacks were not programmed into the game on purpose and many have been proven to cause damage to game cartridges.
There are ways to teach glitch attacks to certain Pokémon without usage of a cheat-code device such as Gameshark, however, most of the time only glitch Pokémon will learn glitch attacks.
The only widely known ones are those for Pokémon Red, Blue and Yellow. The attacks have the same effect on all three versions. 
Also, the move Thunderbolt, when used in some Ruby and Sapphire cartridges, may cause the game to freeze.
There are many different glitch types that are found to be the types of several glitched Pokémon. The most well known are:
- Bird, one of the types of the Red/Blue Missingno.
- 999 (A glitch type), one of the types of 44 and one of the types of the Yellow version Missingno.
- Pokémaniac, the type of the malicious glitch Pokémon ".4"
- [['|) m) ZM Normal (type), the types of the mysterious ? marks glitches of the 2nd generation
- Rival (type),has the name of your rival.
Aside from Glitch City in Generation I, there are several other areas which can only be reached by way of a glitch in the games. Examples of this are areas in the Sevii Islands that are retrievable via their index number pointer, however, do not have any other data.
In the Generation II games, it is possible to get an item called the Teru-sama, the only description of which is a lone question mark. Generation III likewise has a mystery item. Both of these seem to have been placeholders just in case more slots for items were ever needed, something that came true with the Clear Bell in Crystal and the various newer items introduced just in FireRed and LeafGreen and Emerald, which register as the Teru-sama and mystery item if they are somehow moved into Gold and Silver or Ruby and Sapphire. There is also the Seal Bag, a glitch item obtained from the Generation IV GTS glitch.
The Teru-sama is a glitch item, but can be transformed into the GS Ball if it is given to Kurt in Azalea Town to capture Celebi in Ilex Forest. This works in the Japanese, English and European versions of Pokémon Crystal.
Pokémon Colosseum glitches
Infinite Ball Glitch
When in a battle with a Shadow Pokémon, have the first Pokémon that is selected use any type of Poké Ball. With the second Pokémon, go back to the items screen and switch the Poké Ball used with another in the pocket using the Y button. After exiting the menu, proceed normally, and when Wes uses the Ball, the number in the bag will not go down.
Pokémon XD glitches
In a double battle, if a Pokémon grows to a level at which it can learn a new attack after selecting an attack but before actually attacking, and the attack that was selected is deleted to make room for the new attack, the new attack will be used instead of the one that was selected.
- Missingno. and 'M's special stats for the Mew trick vary (see )
- The Yellow version Missingno. is different from the Red/Blue Missingno., but share the same name. For the Yellow version Missingno., using a Pokémon in Method #3 of the Mew glitch with a special stat of 31, 32, 61, 62 , 63, 67, 68, or 69 would make it appear(). Missingno will revert to level 100 if leveled up to over level 255 with Rare Candies or if it fights in battle and wins.
- The Pokémon 3TrainerPoké, Charizard 'M and Q are the only known catchable glitch Pokémon that cannot be directly caught using the Mew glitch.
- The glitch known as ".4" can corrupt saved data if it uses a certain glitch move, making it impossible to continue and forcing the player to start a new game.