Pokémon Red / Blue · Game Boy · 1996 · Impact: Beloved
A precise sequence of steps exploits a flaw in how the game handles interrupted trainer battles, letting players legitimately catch Mew — a Pokémon Nintendo had intended to be unobtainable without special events.
Mew was a last-minute addition to Pokémon Red and Green, squeezed into spare cartridge space by programmer Shigeki Morimoto and never meant to be caught in normal play — it was reserved for official distribution events. But the games contained an exploitable interaction. Certain "long-range" trainers spot the player the instant they appear on screen, and if the player opens the Start menu and uses Fly at the exact frame the trainer’s exclamation triggers, the pending battle is left in limbo. When the player returns to the map, the game immediately starts an encounter using leftover data in memory — and the "Special" stat of the last Pokémon fought determines which species appears. Battle a Pokémon whose Special stat is 21 (such as certain Slowpoke or Shellder) and the wild encounter becomes a level-7 Mew. Unlike cheat-device Mews, the glitch produces a fully legitimate creature, and it spread by word of mouth years before players understood the mechanism behind it.
The glitch relies on two quirks. First, the "Trainer-Fly" bug: some trainers are coded to notice the player from the maximum possible distance, spotting them the moment they walk on-screen. If the player triggers the Start menu on the same step and Flies away, the trainer battle is queued but never resolved, leaving the game in a corrupted state where it force-starts an encounter on return.
Second, the game decides which wild Pokémon to spawn by reading a value that, in this corrupted state, comes from the Special stat of the last Pokémon the player battled. Pokémon are indexed internally, and Mew’s internal index number is 21. A wild or trainer Pokémon with a Special stat of exactly 21 — a common example being a particular Slowpoke or Shellder — causes the game to spawn Mew at level 7. The most reliable documented route uses the Gambler on Route 8 and a Youngster with a Slowpoke on Route 25.