Pokémon Emerald's hex:0400 glitch move, after corrupting the battle style from Single Battle to Double Battle.
A glitch move is a move or attack that was not coded into the game nor learnable by normal Pokémon, but can be accessed through a glitch or a cheating device.
In Generation I
In Generation I, glitch moves may freeze the game, while others have non-freezing effects, such as TM05 raising the user's evasion by two stages in Pokémon Red and Blue, but not in Pokémon Yellow.
Most glitch moves induce Super Glitch (moves hex:A6-C3) when their name is displayed, but if the player works around Super Glitch with the hex:50 tile trick, then it is possible to use them.
In the English versions, glitch moves hex:C4-C8 appear as HM01-HM05. Glitch moves hex:C9-FF appear as TM01-TM55.
The animations of glitch moves may seem a bit odd for their effects.
Obtaining glitch moves
The Cooltrainer move (hex:00) can be accessed with the Ditto move switch glitch.
Certain glitch Pokémon learn glitch moves when leveling up, such as ゥL ゥM 4 learning TM05 at level 10, or 'M (FF) learning the hex:DF Super Glitch move at level 207.
Using the trade evolution move glitch, players can obtain glitch moves by sending a Pokémon with a Generation II only move back to Generation I, for example, Graveler learns Rollout (hex:CD) after a trade evolution into Golem. If this Golem is traded back in the same trade session, Rollout will be converted into TM05.
In Generation II
In Generation II there are only five glitch moves because valid moves range from hex:01-FB in a move byte, leaving glitch moves 00, FC, FD, FE and FF.
All of them can be obtained by trading a valid Generation I Pokémon knowing one of the moves over to Generation II. For example, Ditto's Cooltrainer move will become a hex:00 glitch move in Pokémon Gold and Silver.
Simply opening the fight screen with one of the glitch moves may freeze the game. A certain glitch move in Pokémon Gold/Silver, though freeze inducing has been reported to not freeze the game and work like Substitute under unknown conditions.
In Generation III
Glitch moves can be obtained on certain corrupted Pokémon with the Generation III bit set glitch. Unlike previous core Pokémon games, two addresses define a Pokémon move, meaning that there are a massive 65536-354 (Psycho Boost's index number)=65182 glitch moves.
Some glitch moves cause battle type corruption, for example, move hex: 0400 in Pokémon Emerald changes the battle type to Prof. Birch and double battle mode simply by viewing its name, and this makes it possible to skip Trainer battles by using a Fluffy Tail.
Other battle corruption glitch moves may need the player to use the move first.
Many Generation III glitch moves freeze the game or often if not always miss, but at least one deals damage.
In Generation IV
Glitch moves in Generation IV can be used without freezes through the use of Assist. (source) The hex:01F6 move has been reported to deal damage, and then cause many effects at once, including draining health, causing the user to 'wake up' even if they weren't asleep, cause the user's HP to be sapped, and more.