Pokémon Red / Blue · Game Boy · 1996 · Impact: Data Loss
Encountering the glitch Pokémon MissingNo corrupts the Hall of Fame records in Pokémon Red and Blue, scrambling the saved records of the player's championship teams.
Pokémon Red and Blue record each team that defeats the Elite Four and the Champion in the Hall of Fame, which the player can later review from a PC. MissingNo, the best-known glitch Pokémon, is usually met through the "old man" trick: watching the old man in Viridian City demonstrate how to catch Pokémon, then flying to Cinnabar Island and surfing along its east coast. When MissingNo appears, the game tries to decompress a sprite from invalid data. The decompression routine writes past the buffer it normally uses and into the memory that holds the Hall of Fame records, scrambling them the moment the encounter begins. The damage stays hidden until the player opens the Hall of Fame and finds garbled entries. The same encounter is responsible for the famous item duplication bug, which adds 128 to the quantity of the item in the sixth slot of the bag. Because so many players met MissingNo deliberately, to duplicate Master Balls or Rare Candies, a scrambled Hall of Fame became one of the best-known side effects of the glitch.
When a wild Pokémon appears, Red and Blue decompress its front sprite into a working buffer before drawing it. MissingNo's data describes a sprite with dimensions the routine was never meant to handle, so the decompression writes past the end of the buffer. The area it spills into holds the Hall of Fame records, which are therefore overwritten as soon as the battle begins.
Nothing about this is visible during the battle itself. Players discovered it only later, when they checked the Hall of Fame on a PC and found their championship teams replaced by garbled names and impossible species.
MissingNo became one of the most widely discussed glitches of the 1990s, and Nintendo Power warned players to avoid it. For many young players, a scrambled Hall of Fame was their first direct experience of data loss caused by a bug rather than by a dead battery or a deliberate deletion.
The community's efforts to explain exactly what MissingNo did — which memory it touched, why the item count jumped, why the Hall of Fame broke — were among the earliest examples of systematic, player-driven bug analysis in a major franchise, and that tradition of Generation I glitch research continues today.