Safety & Side Effects

Known Side Effects (REAL)

Lets seperate fact from fiction. These are the confirmed, reproducable side effects of encountering MissingNo:

1. Item Duplication (6th Bag Slot)

This is the big one and its the reason most people seek out MissingNo in the first place. When you encounter MissingNo (you dont even have to catch it, just seeing it is enough), the game adds 128 to the quantity of whatever item is in the 6th slot of your bag.

If you had 1 Rare Candy in slot 6, you now have 129 Rare Candies. If you had 1 Master Ball, you now have 129 Master Balls. The maximum display is x255 but the internal counter wraps around after that.

Technical explanation: The encounter routine has a bug where it reads and writes to a memory address that overlaps with the item quantity data for your 6th bag slot. Specifically, it performs a bitwise OR with 0x80 (binary 10000000), which sets the high bit of the byte. If the quantity was 1 (00000001), it becomes 129 (10000001). This is not a feature. Its a genuine buffer overlap bug.

2. Hall of Fame Corruption

After encountering MissingNo, if you check the Hall of Fame in the PC at any Pokemon Center, youll see corrupted data. Garbled sprites, wrong names, and glitchy graphics scrolling across the screen. It looks wild but its completely harmless.

The Hall of Fame data is stored in SRAM (save RAM) and MissingNo's oversized sprite data writes past its allocated buffer, overflowing into the adjacent Hall of Fame data. Your actual game save, pokemon, and items are stored in a different SRAM region and are not effected.

3. Temporary Sprite Glitches

After encountering MissingNo, you may notice some sprites in the overworld look corrupted. NPCs might look garbled, menu graphics might have weird pixels. This is because MissingNos sprite data (which is basically random VRAM garbage) gets loaded into the sprite tile cache and dosnt get properly cleared.

Fix: This clears itself as soon as you open and close the Pokedex, enter a battle with a normal pokemon, or change areas. Its purely cosmetic and temporary.

MYTHS (NOT REAL)

I have spent years debunking these and I will keep saying it until people listen:

MYTH: "MissingNo deletes your save file"

FALSE. This is the biggest myth in all of Pokemon and it drives me crazy. I have encountered MissingNo on twelve different cartridges over 27 years of research. I have caught it, put it in the PC, battled with it, used it in my party. Not one single save file has been deleted or corrupted in any meaningful way.

What people THINK is save corruption is actually the Hall of Fame glitch mentioned above. They see garbled data and assume their save is ruined. Its not. Your pokemon are fine. Your items are fine. Your progress is fine.

Nintendo themselves contributed to this myth by warning against the glitch in official publications, but they had a financial incentive to discourage players from duplicating items. It trivialized the game.

MYTH: "MissingNo spreads like a virus to other games"

FALSE. MissingNo cant "spread" through link cables. If you trade a caught MissingNo to another game, weird things might happen (it might turn into a Rhydon or another pokemon due to index number remapping) but it does not infect the other cartridge with any kind of glitch virus. Thats not how ROM-based games work.

MYTH: "MissingNo can destroy your Game Boy hardware"

ABSOLUTELY FALSE. Software cannot damage hardware on a Game Boy. The worst possible outcome is a game freeze that requires you to turn the power off and back on. Your Game Boy is fine. The cartridge is fine. The save battery is fine.

MYTH: "MissingNo was put there on purpose by Nintendo as a secret"

UNCERTAIN. This one I cant fully debunk because we dont know what the developers were thinking. The empty index slots are clearly unintentional. They exist because the developers reserved 256 possible pokemon indices (0-255) but only used 151 of them (plus some duplicates for in-game trades). The Old Man glitch that lets you access them is almost certainly a bug.

But the ghost sprite at index 0? The fossil sprites at 182-183? Those feel deliberate. Someone put display sprites on those empty slots. Whether it was a developer having fun or just a memory initalization artifact is something I continue to investigate.

Safety Reccomendations

Based on 27 years of research, here is my offical safety guidance: