The address D05A (D059 in Yellow) manages one kind of 'encounter system', and valid battle systems include a normal wild Pokémon battle (hex:00), the old man attempting to catch a Pokémon (hex:01), a Safari Zone battle (hex:02), the unused encounter system in Pokémon Yellow (hex:03) and the Professor Oak battle system (hex:04).
Index numbers not listed here and hex:03 in Red/Blue are for glitch encounter systems. All of them seem to share the same effect.
A glitch battle system can be accessed with the GameShark code 01035AD0 (Red/Blue) or 01055AD0 (Yellow).
The invalid battle systems work like this:
If the player has no Safari Balls in memory, they will get the "PA: Ding-dong!" message after making an action and the encounter will end, but the player will not be warped to the Safari Zone. No Pokémon is sent out at first. The fight option does nothing. The player can run but has a chance of not being able to escape. If the player accesses the items they will be forced to used the first item, even if it doesn't work in battle, where in that case it causes an infinite loop. If the player heals a Pokémon, or uses an Ether/Elixer, or fails to capture a Pokémon nothing will happen and it won't be the opponent's turn afterwards. If the player uses an 'X' item or Guard Spec a Pokémon's name, level and health points will appear for a short period of time and disappear. Afterwards the opponent doesn't attack. For X items the game will raise the relevant stat if it isn't maxed. If no Pokémon was sent out, the game will say that [player name]'s stats rose. If the player switched to a Pokémon it will be sent out but it will still be the player's turn. If the player switches to the first Pokémon the game will say that a Pokémon is already out. No message appears, and apparently nothing happens if the player tries to switch to a fainted Pokémon and doesn't get the "(x) is already out!" message. If the player opens the Pokémon menu using an item, they won't be able to close it again unless the item is used. The enemy Pokémon will eventually run away.
Accessing a glitch battle system without cheating
Glitch battle system hex:7F can be accessed in Pokémon Red and Blue using the hooked Metapod glitch; which is a sub-glitch of the Yami Shop glitch with one step south of Cerulean City Mart's door as the last location the Pokédex screen, the Pokémon screen or the items pack was opened.
Though glitch battle system hex:22 can be accessed with the hooked Dragonite glitch in Pokémon Red and Blue, it is impossible to press A or B after "The hooked Dragonite" appears, so the player cannot do anything.
Unused encounter system (Yellow)