[waltjp]

I think you also should consider 'how' the 12th player left the field. If a late arriving sub enters the huddle and the huddle breaks immeadiately upon his arrival. The player now leaving the field does so in a manner that does not leave any doubt to his intentions. I don't see this as a penalty under any circumstance. However, if the replaced player now slowly moves toward the sideline and there is doubt whether he's assuming a position in the formation or just slow gettting off I'd flag him.