Missingno. is a glitch type Pokémon in Pokémon Red, Blue and Yellow. This is possibly the most known Pokémon glitch. The glitch occurs due to a bug in the game's programming from the Old Man battle, and others.
The most common way to encounter Missingno. in the Red and Blue versions of Pokémon is to do the following:
- Go to the northern part of Viridian City, and watch the Old Man's demonstration on how to catch a Pokémon.
- Fly to Cinnabar Island
- Surf up and down along the east coast of the island until Missingno. appears. (Depending on the player's name, Missingno. may not show up, and other Pokémon may appear.)
It will increase the quantity of your 6th item by 128 (unless the sixth item is already at 128 or more) when encountered or captured. This can be used to duplicate rare items, such as Rare Candies, Master Balls, and Nuggets. However, battling it will result in a permanently corrupted Hall of Fame. Looking at it's stats will cause the graphics to become inverted, which can be fixed by looking at any pokemon's pokedex entry, preferably shortly after you caught it. This is because the game thinks it needs to display the Pokedex entry for a new Pokemon that doesn't exist.
Why it works
The coding involved in the Old Man fight with Weedle functions much like a random battle code, except that it is geared to change character name and sprite, encounter Weedle, and change back. Since the character name must be saved to be reset later, the programmers used the battle code to save it in. When you fly to Cinnabar and surf along the coast, the coding on the coast does not define what Pokémon to battle, therefore the battle code is not reset. Since the last random battle that occurred set the code to the character's name, the Pokemon encountered depends upon the letters of the name. This glitch is sometimes used to obtain rare Pokemon by changing the character's name to cause specific Pokemon to appear. The reason this causes Missingno. (and sometimes glitched Pokémon Trainers) to appear is that there are actually 256 Pokemon slots although only 151 Pokémon were defined. 256 was used because the variable type for the battle code won't allow a number smaller than 256. The rest of the slots hold 56 other glitch Pokémon and 39 Missingno. (which explains its name "missing number," e.g. an unused slot number). Missingno. is actually the name of an error the game encounters, but since there's no data for the Pokémon about to be encountered, the game uses the data it has.