Unused encounter system (Yellow)
Unused map locations | Early English promotional Pokémon names | Eternal Flower Floette | Bird type | Unused music | Unused encounter system (Yellow) | Unused Bulbasaur slot machine symbol | Unused Shadow Pokémon | Cacophony | Unused Japanese list strings (Generation I) | Unused items | Unused Pokémon data | Unused and redundant Egg moves
See also:
S.S. Anne Ship Truck
An unused battle system exists on Pokémon Yellow with a hexadecimal identifier of 003. When encountering a wild Pokémon the player cannot fight the opponent because the game will display the message "Hurry, get away!" 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, however the wild Pokémon will not make a move. If the player selects run they will successfully escape from the battle.
It is similar to the actual hexadecimal identifier used when Professor Oak attempts to catch a Pikachu at the beginning of the game; however, the player's name is not temporarily changed to 'PROF.OAK' and neither is the player's sprite changed to that of Professor Oak. Nintendo also decided that Professor Oak will catch a special Pikachu, therefore Nintendo chose to make all Pokémon encountered through this variable to have Pikachu's cry but in the unused battle system all Pokémon have their normal cry.
It could be suggested Nintendo at first planned the player to catch the Pikachu by themselves but technical limitations would make it impossible for the Pikachu to have a special cry when encountered. If the special pikachu was made to occupy its unique index number it would be difficult to restrict it from being traded to Pokémon Red/Blue into a glitch Pokémon.