Unused encounter system (Yellow)
Three distinctive types of battle systems exist in Pokémon Red/Blue and Pokémon Yellow which were never used in the final game. One of these unused battle systems appears to be beta element as it gives the message "Hurry,get away" for all Pokémon encountered by it however, others have mild glitch effects, such as the values occupying slots 005 to 255.
Beta battle system(Pokémon Yellow only)
A stable battle system exists for slot 003 on Pokémon Yellow. When encountering a wild Pokémon the player cannot fight the opponent because it will give the message "Hurry, get away." If an item is used the game will automatically select the first one similar to how Professor Oak does to catch a Pikachu at the beginning of the game. Pokémon can be switched but the wild Pokémon will not make a move. If the player selects run they will successfully escape from the battle.
Unlike slot 004 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 slot 004 to have Pikachu's cry but in slot 003 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.
Slot 003 equivalent (Pokémon Red/Blue only)
This battle system has the equivalent hexadecimal identifier to Pokémon Yellow's. However its effects appear to be purely glitch or as a value to account for coding elsewhere. When encountering a wild Pokémon it will appear at level 0 by default which may suggest that it is reading data from another byte than the ordinary one which reads the Pokémon's level. When 'FIGHT' is chosen the battle will suddenly end but then instantly start again. This is also the case for other battle options for example if 'item' is chosen the battle will suddenly end after an item is used. Like its Pokémon Yellow equivalent the player will not send out any Pokémon and instead the player's battle sprite for trainer battles will remain on the screen.
Slots 005 and onwards up to 255 (starting from 004 in Pokémon Red/Blue)
The rest of the slots corresponding to unused battle systems share the same effect. For these battles when the player attempts to choose 'RUN' the game will always display 'Can't escape.' Like slots 003 on both games the player will not send out any Pokémon and will keep the normal trainer battle sprite throughout the rest of the battle. When the player selects 'FIGHT' nothing will happen as although the normal selection sound is made the game will not progress to the move selection screen.