In Pokémon Red and Blue, the MissingNo. glitch added 128 items to the sixth inventory slot.
MissingNo., short for Missing Number, is a glitch rather than an official Pokémon species. Players could trigger the encounter by combining the Viridian City capture demonstration with travel to Cinnabar Island and surfing along its eastern coastline.
The glitch exploited how the games stored encounter data. After the tutorial, the player’s name could remain in a data buffer, and Cinnabar Island did not fully overwrite that information before generating wild encounters. Invalid data could therefore produce MissingNo.
The encounter could also cause visual corruption and affect the Hall of Fame. Nintendo warned that the glitch might damage game data, although players widely discussed its item-duplication effect. The sixth-slot increase is a specific behavior of the original Red and Blue programming, not a standard Pokémon ability.