mew-glitch

1996. Mew sits in the code of Pokemon Red and Blue and nobody is supposed to be able to catch one, so players go hunting for holes. The one they find: step into a trainer's line of sight and escape with Fly before the battle starts. The game leaves that fight marked as still pending. Open the menu somewhere else and the pending fight goes ahead, using whatever number happens to be in the special stat as the species. Twenty one gives you Mew.