When the player didn't have the intention of leaving, when it's not his fault.
Also when host bans a player he shouldn't have banned.
If 40% of users in game complain about unplayable conditions (with !rmk) in the first 10 minutes of the game, all players have to leave (and game owner can rehost the game).
If the owner bans people under that condition, that's ban abuse.
Also if a player leaves in the last few minutes of the game, the rules state it's a warn. If the host bans people leaving a few seconds before the game ends that would also obviously be ban abuse.