One of the most popular oddities in Pokémon, and gaming history in general, is the presence of a mysterious, glitchy non-Pokémon called "MissingNo."
Known in Japanese as けつばん ("Ketsuban", literally "Missing Number"), this placeholder is used to fill the empty entries in the Pokémon ID list. In the localized versions Red and Blue, there are 39 instances of the string "MissingNo.", each used by an empty entry in the internal index table. However, in the Japanese games 38 of the entries use the name Ketsuban, with the final blank entry instead using the name ゴースト (Ghost) and the Pokémon Tower ghost sprite.
Out of all of the instances of MissingNo., 36 of them use interpret garbage data when displaying a sprite, and due to the abnormally large size of this garbled graphic, viewing it causes the player's Hall of Fame data to become corrupted. However, the final three MissingNo. have legitimate sprite data.
Each MissingNo. is numbered #000 in the Pokédex, and since a Pokémon's base stats are ordered by Pokédex number rather than internal index number this results in all 39 MissingNo. sharing the same basic information, including types, stats, moves, and Pokédex data. However, none of this data is actually valid, as the game is actually reading data far beyond the boundaries of the base stats table, misinterpreting data used to define the parties of trainers as base stats. The Pokédex entry for MissingNo. defines it as a "??? Pokémon" with a height of "1m" (3.3 ft), a weight of "10kg" (22.1 lb), and a placeholder Pokédex entry reading 「コメント さくせいちゅう」 ("Comment under construction"). However, this information was not translated or localized in the English versions of the games and became garbled, resulting in the Pokédex entry displaying an erroneous height of "10ft" (3.1 m) and a weight of "3507.2lb" (1590.8 kg).