Missingno resulted from the interaction between a programming shortcut in one of the game's first locations and an encounter code error in one of the final areas of Pokémon Red and Pokémon Blue.
In Viridian City, an old man demonstrates the process for capturing Pokémon. The game needs to temporarily replace the player's name with the name "OLD MAN," and has to store the player's actual name somewhere. Viridian City has no grass tiles, so the programmers used the portion of the memory normally allocated to determining random Pokémon encounters on grass tiles as a temporary storage spot.
Cinnabar Island, which appears much later in the game, hosted a coding oversight along its eastern shore. Water tiles lacked the normal encounter probability code, meaning they did not overwrite old encounter data when entered by the protagonist. Players arriving from the sea encountered the usual ocean enemies, so the bug was very easy to miss.
When the player finished watching the old man capture a Weedle, they could then use a Pokémon's Fly ability to move directly from Viridian City to Cinnabar Island. From there, the player could access the aforementioned water tiles and encounter wild Pokémon. The game still had the player's name in the random encounter memory, so it incorrectly attempted to load this data as an enemy Pokémon. Based on the third, fifth, or seventh letter in the player's name, it could produce a number of absurdly high-level Pokémon or a collection of unrelated sprite fragments. The most popular of the latter displayed the name MISSINGNO, which was short for "missing number."
The freakish abomination of poképarts was hardly cute and was by no means the last glitch-based Pokémon, but it stuck with the audience because it filled the gameplay void left when players finally caught'em all. Missingno represented a new challenge beyond the limits of the official list. Players eager to obtain Mew, the game's official secret Pokémon, needed to attend official events. Missingno, by contrast, was an extra Pokémon that anyone could get regardless of age or location. The fact that it registered in your Pokédex as Pokémon #000 just added to the sense of mystery.
Gamers are constantly looking for hidden secrets and new ways to enjoy their favorite games. The search for new experiences brings players back to games they've finished and glitches often provide some of the best unscripted easter eggs. Games grow more complicated with every sequel and quality control can't possibly find every interaction between the game, its downloadable expansions, and the curiosity of a generation of problem solvers. YouTube, Wikipedia, and countless web forums ensure that if the Spy can walk like a crab, you'll know about it.
William Bloodworth has written short stories, staffed anime conventions, and spent too much money on Magic: the Gathering. You can hear his nerdy ramblings at www.flavortextpodcast.com.