Pokémon Red and Blue / GAME HISTORY

Why Can MissingNo. Change the Sixth Item in Pokémon Red and Blue?

The sixth item changes when an out-of-range Pokédex seen flag reaches the bag’s quantity byte.

By Patchlo Editorial Desk · Published October 24, 2026 · Sources checked October 24, 2026

Scope: English Pokémon Red and Blue for Game Boy. Other releases and glitch species can behave differently.

Why does seeing a Pokémon touch the inventory at all?

Normally, it does not. The game records an encountered species in a compact array of Pokédex flags. The battle code converts the species index to a Pokédex number, subtracts one, and sets the corresponding flag.

MissingNo. is outside the normal species list. That path can turn its zero Pokédex number into the out-of-range flag index 255. The write then reaches beyond the space reserved for ordinary seen flags.

Why the sixth slot rather than a random item?

The Red/Blue disassembly places the seen flags, bag item count and bag entries next to one another in working memory. With that layout, the out-of-range write reaches the sixth item’s quantity byte.

Setting its high bit adds 128 when that bit was previously clear. If it was already set, setting it again does not add another 128. This is a bit being set in a stored quantity, rather than the game awarding a stack through its normal item logic.

Is the Old Man trick the same bug?

It helps arrange the encounter, but it is not the inventory write itself. The well-known route through the catching demonstration and Cinnabar’s coast explains how unusual encounter data can be reached. The seen-flag path explains why a bag quantity changes afterward.

Keeping those steps apart avoids a common bad explanation: the player’s name or the scrambled battle picture does not directly become a pile of items. The screenshot shows the encounter; the code and memory layout explain the sixth-slot effect.