Unused encounter system (Yellow)
An unused encounter system exists in Pokémon Yellow with an index number of 03. It is accessible with the code GameShark code 010359D0 and does not exist in Pokémon Red and Blue.
When this battle system is in action, the player's 'backsprite' will appear but no Pokémon will be sent out.
The player cannot fight the opponent because the game will display the message "Hurry, get away!" (Japanese: はやく にげるのじゃ!, roughly: run away quickly!).
If an item is used the game will automatically select the first one similar to how Professor Oak attempts to catch a Pikachu at the beginning of the game. Pokémon can be switched out in this battle system, however the wild Pokémon will not make a move. If the player selects run they will run away from battle, apparently with a 100% flee rate. It is possible to flee from Trainer battles with this encounter system just by selecting 'run'.
It is possible that this battle system was meant for encountering a Pokémon without having any, but it was not implemented for this into the final game, and battling with no Pokémon will normally trigger the wild appeared! effect.