MissingNo could multiply an item in Pokémon Red and Blue

When the Coast Became a Glitchy Lookup Table

In Pokémon Red and Blue, the narrow strip of water beside Cinnabar Island was not supposed to produce a new Pokémon. But the games could use encounter data left over from another part of the game when calculating what appeared there. Surfing along the coast made the game interpret that stray data as a wild encounter. The result was MissingNo.—short for “Missing Number”—a creature-shaped sign that the game had read an invalid Pokémon entry.

Its blocky, scrambled appearance was not a deliberately designed monster. The game tried to draw a sprite for a number that had no proper Pokémon assigned to it, so it displayed whatever image data happened to be available.

Why an Encounter Could Change Your Items

The stranger effect came from how the original games organized information in memory. A Pokémon’s identity, its picture, and the player’s inventory were represented by numerical data. When the game handled MissingNo., it could mishandle nearby data rather than keeping each piece neatly separate.

In the familiar version of the glitch, the quantity of the item in the sixth slot of the player’s bag jumps by 128 after an encounter. Players could put a valuable item there first, turning an accidental memory error into a duplication trick. The item was not copied through a normal game feature; the glitch altered the number the game used to track its quantity.

A Bug That Became Part of the Game’s Story

The coast exploit depended on a chain of hidden conditions: a particular game state, the edge of the map, and an encounter the game did not know how to represent correctly. That made it feel less like finding a secret room and more like peeking through a crack in the software’s logic.

For kids experimenting with the game, the lesson was concrete: changing where the game looked for data could change what it believed was in the world—and even how many items were in a bag. MissingNo. turned a technical mistake into a playground experiment, with a stack of rare items as the visible result.