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Pokémon Red and Blue shipped with a glitch creature, MissingNo., that bumps the sixth item in your bag by 128 and scrambles the Hall of Fame...
Pokémon Red and Blue shipped with a glitch creature that Nintendo ended up warning people about. Wikipedia says encountering MissingNo. increases the number of items in the sixth entry of the player's inventory by 128.
Smogon says the sprite will be large enough to overflow into the Hall of Fame data. One encounter can add items and damage a save in the same visit.
If you still own a Red or Blue cartridge and want to try it, do it on a save you don't mind losing.
If you write code that looks things up, decide what should happen when the lookup finds nothing before a player decides it for you. Players will find the gaps between your valid inputs.
Screen Rant reports that Nintendo called the glitch a programming quirk, and Wikipedia says Nintendo warned that encountering it may corrupt players' game data.
The cause is a gap in the map. Kotaku explains that the Cinnabar Island strip is programmed such that things can spawn there, but not programmed to know what specific Pokémon should spawn there. That gap is where MissingNo. lives.
The corruption comes from the creature's picture.