From Glitch City Laboratories
An unused battle system exists on Pokémon Yellow with a Hexadecimal Identifier of 03. If this battle system is activated through the use of a game-altering device 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 in this battle system, however the wild Pokémon will not make a move. If the player selects run they will successfully escape from the battle.
Unlike the actual hexadecimal identifier used when Professor Oak attempts to catch a Pikachu at the beginning of the game, the player's name is not temporarily changed to 'PROF.OAK' and neither is his or her in-game sprite. 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 default cries.
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 a unique index number it would be difficult to restrict it from being traded to Pokémon Red and Blue into a glitch Pokémon.
- Glitch battle systems
-  An article on the forums about the unused battle system
-  A Youtube demonstration by ChickasaurusGL