The Zero maximum HP glitch is a recurring glitch in Generation I and Generation II core series Pokémon games.
If a Pokémon has zero maximum HP (but not 0/0 HP), attempting to view the party or its summary may result in a game freeze.
The game will lock up:
If the user's Pokémon:
- After "Wild (POKéMON) appeared!" even if the Pokémon isn't in the first position.
- After "Go! (x)" if the player encountered a Trainer and the Pokémon is in the first position.
- After opening up the party.
- After switching the relevant Pokémon. (can be avoided if an above 0xFF hides the Pokémon)
If the opponent:
- After "Wild (POKéMON) appeared!".
- After selecting a move.
- After closing Fight, Item or PkMn. If PkMn is closed when the lock up occurs the party's menu sprites become invisible.