The Glitchhouse · Pokemon Red and Blue

The item duplication glitch

One glitch encounter on the Cinnabar coast reaches into your bag and adds roughly 128 to your sixth item's count. Put a Rare Candy there, or a Master Ball, and walk away rich.

Decoded from a Pokemon Red ROM · part of The Glitchhouse

Your bag is a simple list: twenty slots, each one an item and a number for how many you hold. Surf the east coast of Cinnabar Island and you run into the same broken encounter that draws MissingNo, and as a side effect it adds a big number, near 128, to the quantity of whatever sits sixth in your bag. Nothing else changes. So you arrange the bag once, trigger the encounter, and the sixth item's count jumps from a handful to a stack taller than the game meant to allow.

It is always the sixth slot

The glitch does not care what your sixth item is. It reaches to that one fixed position in the bag and adds to its quantity, whatever happens to be sitting there. So the whole trick is arranging your bag first. Tap any item to move it into the sixth slot and watch what the glitch would leave you with.

Your bag is twenty slots of an item and a count. The Cinnabar coast encounter always adds about 128 to the count in slot six. Put a Master Ball there and you get a stack of them; put a Rare Candy there and you get instant levels. The slot is fixed; the item is your choice.

The trick, step by step

The setup is the same broken encounter that makes MissingNo, worked out and shared by the glitch community decades ago. Arrange your bag, then run this route. Tick each step.

The old man's demo leaves your name in memory where the wild data should be; flying to Cinnabar and surfing its east coast reads that leftover, spawning MissingNo or 'M and, as a side effect, adding about 128 to your sixth item. You do not even have to fight it. Run, and check your bag.

Why about 128

A quantity in the bag is a single byte, eight bits, and the glitch does one clean thing to it: it switches on the very top bit, worth 128. It does not add and it does not roll over, it just forces that one bit on. So if the bit was off you gain exactly 128, and if it was already on, worth 128 or more, nothing happens at all. Slide your starting count and watch the byte.

BEFORE
SET TOP BIT →
AFTER
Because it only forces the top bit on, the glitch cannot push a stack past 255, and it cannot touch a stack that already holds 128 or more, exactly what the glitch community found in practice. The biggest useful result is a full 255: set a count to 127 (0111 1111) and one dupe makes it 1111 1111. That is the basis of the classic dupe, toss two, dupe again route to 255 documented by MissingNo.Xpert.

So what do you clone

A stack of about 129 of the right item breaks the game wide open. Here are the classics. Pick one and see what it buys you.

Rare Candies for instant levels, Master Balls to catch every legendary, Full Restores so you never lose, or Nuggets to sell for a fortune. One trip to the coast, one item in slot six, and the economy of the whole game is yours. Choose kindly, it is your save.

It is the same bug as MissingNo

The item duplication is not its own glitch. It is a side effect of the exact encounter that draws MissingNo, one broken read on the Cinnabar coast with two consequences. Flip between them.

See MissingNo in the Glitchhouse →
The coast has no real water-encounter data, so the game reads your name where the wild Pokemon should be. That single mistake both spawns MissingNo on the screen and adds about 128 to your sixth item behind the scenes. Meet the specimen on its own page.

Is it safe

Mostly yes, and the reputation is worse than the reality. The duplication itself never touches the save. There is one honest caveat, and it is cosmetic. Tap each action to see the risk.

The item duplication and running from MissingNo are completely safe. The only documented side effect is that meeting MissingNo can scramble your Hall of Fame record, the replay of your first championship. It is graphical, it does not brick the save, and viewing a normal Pokemon fixes any garbled sprites.

Does it still work

The +128 is baked into the game's code, so it runs anywhere that code runs. The only wrinkle is the version: the classic coast route is a Red and Blue thing. Check where you are playing.

The sixth-item bump is present in every original Red and Blue, the 3DS re-release, and every emulator. Yellow reworked the Cinnabar encounter, so the classic route does not line up the same way there. Everywhere it does work, the trick is identical.

From the ROM: why the sixth slot, why 128

Here is the part most guides skip, and it is the whole reason the trick works. The duplication is not a special item routine at all. It is the game marking MissingNo as seen in your Pokedex, and missing the table by a mile.

The Pokedex "seen" flags are just a row of bits, one per Pokemon. To set a bit the game takes the Pokemon's Pokedex number, divides by eight to pick a byte, and switches on the matching bit. MissingNo's Pokedex number is far larger than 151, so that math walks right off the end of the seen array, past the item counter, and lands inside the bag, exactly on the sixth item's quantity byte. The bit it flips is the top one: value 128.

; ram/wram.asm  (these sit back to back in memory)
wPokedexSeen::   flag_array NUM_POKEMON  ; 1 bit per Pokemon, only 151 exist
wPokedexSeenEnd::
wNumBagItems::   db
wBagItems::      ds BAG_ITEM_CAPACITY * 2 + 1 ; (item, quantity) x20; slot 6 qty is byte 11

So "sixth item plus 128" is not a rule someone wrote, it is where MissingNo's out-of-range seen bit happens to fall. That is also why it only ever sets the top bit, why a stack already above 127 rolls over instead of growing, and why moving a different item into slot six hands the bonus to that item instead. The bag never knew it was hit.

Where to go next

The duplication is one thread in a knot of Cinnabar-coast glitches. Pull the others.

MissingNo.The causeThe garbage Pokemon whose encounter powers this whole trick, decoded on its own page. The Mew glitchA leftover PokemonAnother famous glitch that reads a stray number, this time as a real, catchable Mew. The GlitchhouseEvery specimenThe full collection of Gen 1 glitches, each decoded from the ROM and made playable.

Go deeper: the Name Glitch tool | the Gen 1 battle engine | browse the Glitchhouse.

Go deeper: MissingNo. | the Mew glitch | browse the Glitchhouse.