MissingNo.

MissingNo. (けつばん), short for "Missing Number" and sometimes formatted MissingNo or MissingNO, is a Glitch Pokémon found in Pokémon Red and Blue Versions and Pokémon Yellow Version. As it is an unofficial Pokémon, MissingNo. is only encountered through glitches, and is among one of the most famous video game glitches. MissingNo. does not appear in any later Pokémon games, although similar Glitch Pokémon have appeared.

To capture MissingNo., the player character must watch an in-game tutorial for Pokémon capture in Viridian City. Second, they must use a Pokémon with the Fly move to instantly travel to Cinnabar Island. They must then use a Pokémon which knows Surf to travel up and down the eastern shore of the island until a MissingNo. appears.

These events manipulate the game's random encounter system to generate a Pokémon with an invalid identifier. Each area within the game assigns values to a data buffer to represent the Pokémon that can be encountered in that area. Some areas, such as Cinnabar Island, do not overwrite the data in this buffer, so the data from the previous area is used instead. During an in-game tutorial in which the player character's name is overridden to read "Old Man", their actual name is temporarily copied to that same data buffer. If the player character travels directly to Cinnabar Island after viewing the tutorial, their name reads as the Pokémon that can be randomly encountered in that area. Because the player character's name is not intended to be read as this kind of data, the game can attempt to generate an encounter with a Pokémon with an invalid identifier, such as MissingNo.

As with any Wild Pokémon, the player character may flee from, fight, or capture MissingNo. After an encounter with MissingNo., the quantity of the sixth item in the player's inventory is increased by 128, and the game's Hall of Fame Pokémon gallery becomes glitched. This beneficial effect resulted in the glitch's coverage by strategy guides and game magazines. Nintendo of America first documented the events that cause MissingNo. to appear in Nintendo Power volume 120 (May 1999). The publication warned that "any contact with it could easily erase your game file or disrupt your graphics".

A captured MissingNo. is functional as a Pokémon and appears in the Pokédex as number 000. It is classified as a hybrid Bird/Normal-type Pokémon; the former type was scrapped from the games before release. It commonly appears with a scrambled block-like form commonly described as a "backward L-shape", but depending on the player character's name, MissingNo. can also appear as one of three ghost or fossil sprites not used by other Pokémon. Temporary graphical glitches may also occur, which can be removed by viewing the statistics page for another non-glitched Pokémon or resetting the game.