When you only have 1 move left and in that last move you are able to kill the last of the monsters. it requires you to spend an additional move token in order to consider the level complete, even though it's already complete. (potential fix) - Just need to hold the buy more moves screen a half second longer so the game has time to run a complete level check again, and make sure it does run a complete level check before the buy more moves screen pops up. May not be a bug, but when you have a colored orb and you complete a row of 5, it should in my opinion give you a grey orb for the 5 stack, but it doesnt. sometimes when buying extra moves it just doesnt do it and fails the level fully.
