Unused encounter system (Yellow)
An unused encounter system exists in Pokémon Yellow with ID 03. It is accessible with GameShark code 010359D0 or by exploiting a complex glitch known as Text pointer manipulation mart buffer overflow glitch.
When triggered, Red's back sprite will appear but no Pokémon will be sent out. The FIGHT option will yield the message "Hurry, get away!" (Japanese: 「はやく にげるのじゃ！」, roughly run away quickly!).
If ITEM is chosen, the game will automatically pick and use the first item on the Bag in a similar fashion to the Professor Oak tutorial at the beginning of a journey where he catches Red's Pikachu. Pokémon can be switched out using PKMN in this battle mode but the wild Pokémon will not make a move. If the player selects RUN, they will always successfully escape, even in Trainer battles.
It is possible that this battle mode was meant to replace "Wild appeared!" while encountering a Pokémon without having any.