Mimic empty moves list lock-up (Generation I)
On either player's side, if Mimic is used and the opponent doesn't have any moves, the game will lock-up after the "(Pokémon) used Mimic" message with the music still playing. However, this doesn't typically apply to a Pokémon manipulated with the swapping Transform moves glitch in Red/Blue (i.e. have Ditto or Mew use Transform, swap the order of moves, and run) because technically Transform is still in the moves list (if there was a move 2 swap with move 1, it would be in move 2 with the 0x00 move above it); in this case Transform will be copied and the lock-up will not occur.
This glitch applies to Red/Green/Blue/Yellow if you can use a glitch to obtain a Pokémon with no moves at all (without using the swapping Transform moves glitch or possibly without using a Select glitch which still leaves a move hidden in the moves list). Additionally, in Gold/Silver/Crystal the behaviour of Mimic was changed. Instead of bringing up a list of moves for the player to copy, it can only copy the last used move. While I'm not sure if a lock-up is still possible, when I modified the moves of a wild Pokémon to have no moves, Mimic would just fail and the opponent would use Struggle.