By forcing the game to swap one set of data with another it turns your only Pokemon into the anomaly stored at hex value 'FF' and also shuffles around other data in the game, namely the door location data.
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From this point the glitch is active, you have sacrificed your only Pokemon to the gods of the underworld, and every 'cycle' of the game (every 4 steps taken) will cause the warp point of every door to change to another location. You then must press Select and act like you want to change its place in your item list except rather than swap an item you attempt to exchange it with the first Pokemon in your party (which also requires use of the Select button). You knew #'s 1 through 151 better than your own family, so what in God's name is this abortion?! It has a terrifyingly alarming name ('MISSING' appears in it), its graphic looks like the bowels of technological hell, and while it cries and fights like a regular Pokemon you have never seen anything like this before in your life. MissingNo's familiar 'L' shaped appearance, non-threatening battle cry, memorable moveset, and item duplication properties are quite vanilla today, but how would you have like to have been the first person to stumble across this beast? Can you imagine minding your own business when suddenly a random encounter begins, the screen freezes black for just a few seconds, and then 'Oh god oh god oh god oh god.' Any self-respecting Pokemaniac in the 1990′s knew the entire Pokedex (and the PokeRap) like the back of their hand. Glitches in future generation Pokemon games are called 'MissingNo' even though they're completely different entities it's a glitch that became the name for every other Pokemon glitch because that's what happens when you're #1.