Revealing mistake: As the funeral of young William's father and brother is over, a man lifts a shovelful of dirt and empties it onto the open grave of one of the men. As he does this you can see the fake body that is wrapped up bounce pretty hard, revealing that it is not in fact a real person but something made out of lightweight material.
Revealing mistake: At the end of the battle of Stirling we see a Scotsman stab something with his sword and we can hear "Ouch.". However there is no one on the ground where he stabs. He just rams the sword into the ground. Then he does it again and we hear "Ouch.". Quite fun to watch. (01:27:55)Mortug
Revealing mistake: When his wife gets attacked, Wallace jumps over a roof and you can clearly see him wearing modern black briefs.
Revealing mistake: During the first large battle between the Scots and the English, there is a clear, quick shot of an English soldier getting quite eviscerated on the right side of his body. For a fraction of a second, you can see a couple drops of blood splatter against some sort of protective glass between the camera and the soldier.
Revealing mistake: In the scene where Wallace comes back (right after his wife is killed), he throws two archers out of the tower. The second archer is supposedly killed on the palisade wall; but right before the camera changes, you can see the wall bending as he lands on it.