Blame @ZombieHam for this.

SKILLS:
1) When a pokémon wishes to learn a new move (due to levelling up) but already has four moves, roll:
   1: The move is not learned.
    2-3: Roll again:
       • 1-4: Replace the corresponding move. (re-roll if impossible [HM])
       • 5: The move is not learned.
       • 6: Replace the move of your choice (or don’t learn it, if you prefer).
    4-6: Replace the move of your choice (or don’t learn it, if you prefer).

2) When attempting to teach a move to a pokémon using an HM or TM, and the pokémon already has 4 moves, roll:
    1: Randomly select a different party member that is permitted to learn that move. Start this process over, except you are now attempting to teach the move to the new pokémon.
    2-3: Roll again:
       • 1-4 Replace the corresponding move. (re-roll if impossible [HM])
       • 5: If a TM, throw it away unused. If an HM, start over.
       • 6: Replace the move of your choice.
    4-6: Replace the move of your choice.

Yes, this is horrible. You are training horrible little monsters who do not listen to you, and who get in the way when you’re trying to teach new moves to other party members. As a trainer, your job is to embrace anarchy and roll with the punches; to survive, you must adapt to the capricious whims of fate (and little bastard monsters).