The 99 item stack glitch is a memory corruption glitch which occurs in Pokémon Red and Blue and Pokémon Yellow (Japanese). It does not work in English Yellow.
If the player performs a task which adds to the number of items in a list (purchasing an item from a Poké Mart, depositing it via the PC or withdrawing it via the PC) while they have an existing stack of that item ×99 in the last slot of the inventory (e.g. Ultra Ball ×99), the game will try to add to an item quantity of the same item from beyond that slot in an attempt to contain the items in an existing stack without creating a new one.
Due to a glitch, the end terminator is ignored if it directly proceeds an item quantity, and the game may add to the quantities of items even if the 'item quantity' represents an unrelated memory address.
- In English versions, when buying Ultra Balls with Ultra Ball ×99 in the last slot of the inventory, it is possible to corrupt $D35D (item 32 quantity; controlling the screen palette) if $D35C (item 32; current sound bank value) is 2 (Ultra Ball).
- In English versions, one can exploit the Yami Shop glitch to buy Master Balls. If D364 (Block-X ID) is 01 and there are no Master Ball stacks which aren't ×99 before it, then one can add to D365 (map exit warp) to access locations up to hex:63 (S.S. Anne Deck). It may be necessary to change the quantity in address D358 to 99 first.
- In Japanese versions, when buying Great Balls with Great Ball ×99 in the last slot of the inventory, it is possible to corrupt number of badges ($D2D5) if the options byte is equal to 3 (Battle Animation On, Battle Style On, medium text speed).
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