>>54754071Related to it, the item duplication glitch works because the catching tutorial temporarily overwrites the player's name with "OLD MAN", so the game has to store the player's name elsewhere until the tutorial is done. It thus stores it in the same memory location as the grass encounter table. This isn't normally a problem, as when you enter a location that has grass, the temporary data is overwritten by the new encounter table, but if you keep the name data loaded and surf along Cinnabar, the Pokemon encounter calls will pull very odd data instead, including missing slots. That's when you encounter MissingNo., or its cousin 'M.
The reason they duplicate items is because every Pokemon species has two values to track whether the player has seen the species, and caught the species, as you can see in the Pokedex. As it happens, MissingNo.'s and M's seen value is located at the same memory as the 6th slot in your inventory. If the item there is less than 128 in number, the game thinks you're seeing MissingNo./'M for the first time, and increments it by 128 to indicate you've seen it now. You can then use those items and do it again and again.
