Freeze status 0 PP move selection glitch (Generation I)
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If a slower Pokémon gets frozen before it gets a chance to use a move, but a move was selected, and then if the faster Pokémon defrosts it with a Fire-type move it will use the last move that was selected.
If the slower Pokémon was switched out while frozen and is switched back in, and then the faster Pokémon defrosts it, it will use the the topmost move in the move list on one side even if it had 0 PP and may use another move on the other.
This allows the PP of any move to underflow to 63 (in this video Psychic was at the top position and had its PP underflow from 0 to 63). Unfortunately these conditions may only be met in a link battle, so in at least Yellow, the 63 PP effect is lost when the battle ends.
Theoretically the Cable Club escape glitch can be used to keep the underflowed PP, but there are two 'better' methods of PP underflow, yet they are restricted to Hyper Beam and partial trapping moves. https://www.youtube.com/watch?v=I8AzgGoJbTs
Also, I haven't tested having hex:FF in the top position for player 1. Perhaps that could make the 63 PP be kept.