From Glitch City Laboratories
Zero Error is a minor glitch in Pokémon Red, Pokémon Blue and Pokémon Yellow. When this glitch is performed the player would have caused a route to become glitched, this will display a dialog box with the words "0 Error."
1. The player must go to Vermillion City.
2. Walk up to the route north of Vermilion.
3. From here, perform a Mew Trick variation by flying away from the trainer in the upper left hand corner of the route.
4. Choose to fly back to Vermilion City.
5. Walk to the route that is east of Vermilion.
6. Fight any trainer here. It does not matter who the player fights.
7. After the trainer battle, the player should go back to Vermilion and talk to anyone there.
8. The player should then walk up to the route north of Vermilion one last time and the game will display the words "0 Error."
The effects of this glitch can vary, from walking and hearing the Jr. Trainer, to seeing the words "0 Error" after pressing Start. This has no other effect on the game, and the player can Fly away to reset the route.
- This glitch can be done on any route where the player can fly away from a trainer before he or she battles him or her. The player would have to do the above steps and walk back onto the original route.