Other mistake: In one of the very last scenes of the movie, when William's friend Hamish takes out William's sword to throw it as a sign of honor for William, when Hamish extends his arm back to throw the sword he only takes a step or 2, but he does not hit any of the men standing in formation directly behind him with this 5 foot long sword.
Other mistake: After the fight at Falkirk where Robert The Bruce is walking through the field, there is a woman walking checking the bodies. All these people clearly are supposed to be dead. The woman lifts the head of a man and then puts it down. After she lets go of the head, the boy picks up his head and looks to his side.