The most common I have seen is bonus actions. Some new players assume everyone gets one and they can do something as a bonus action. Others assume they can do as many bonus actions as they have available or want to use. Bonus actions are poorly named and must not be well explained because I see the same mistakes and confusion over and over.
If it were up to me, I would either use something like the PF2 action economy, or create action categories that are universal to all characters (complex, simple, whatever) and make specific abilities or spells those types of actions.
Oh, you get one fast/simple/immediate/whatever action per turn, and second wind is one of those. Easy peasy.