From Bulbapedia, the community-driven Pokémon encyclopedia.
Glitches are anomalies in a video game; 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.
A glitch Pokémon is a Pokémon that exists in a game due to a programming or level design oversight, and as it was intentionally placed inside the game by Nintendo or Game Freak, it might be hazardous to data in your game. Glitch Pokémon are to be avoided due to the risks they may pose to a player's save file.
Glitch Pokemon List
A Glitch attack, or glitch move, is much like a regular attack except the results are often undesireable.
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 other glitches in the games such as Glitch City, a broken variation of Fuschia City, accessible in Pokemon Red, Blue and Yellow.
Bird type is an element type which occurred due to a glitch.
There is also the Mew Glitch, which allows players to capture Mew without modification.