In Pokémon Red and Blue, encountering MissingNo. increases the sixth inventory slot by 128 items.
MissingNo. is a programming glitch rather than an official Pokémon species. Players can trigger the encounter by watching the Old Man’s capture tutorial in Viridian City, flying to Cinnabar Island, and surfing along the island’s eastern coast. The game can then interpret leftover data as an invalid wild Pokémon identifier.
The inventory effect is one reason the glitch became famous. The quantity of the sixth item is increased by 128, potentially giving players a large stock of useful items. Encountering MissingNo. can also create graphical anomalies and alter the Hall of Fame display. Nintendo warned that the glitch could disrupt saved data or produce other unwanted effects.
MissingNo. is commonly mistaken for a hidden, legendary Pokémon. It is not part of the intended Pokédex and is officially a programming error. Its name is short for “Missing Number,” and the glitch remains accessible in the original games and their Nintendo 3DS Virtual Console releases.