Switching profiles on the main menu will reset Master volume to default levels even if both profiles have a non-default volume setting.
Healing a downed Ally for 0 HP makes them playable at 0 HP and functionally immortal. You can do this by using a healing scroll on a downed character with -1 on a heal spell/scroll and rolling 1.
It's possible for an attack of opportunity to consume an action without the player performing the action. This usually happens when the attack of opportunity hits when the players action has started its animation and the animation is interrupted.