Heart refills not being used up for extra XP.
After you finish a lesson, you can press "refill heart" to regain a heart. You gain the bonus XP from the extra heart, but the heart refill is not used up. If you refill it at the end of the lesson to get 3 hearts, you do not get a lingot. In short, if you press the heart refill button after you finish the last exercise, you gain an extra heart and extra xp, but no lingot if 3 hearts and it doesn't get used up.