Revealing mistake: After Wallace kills the Scottish noble in bed, he jumps out of the castle window into a river on a horse. As he is falling, you can see that the horse is a model and when they fall into the water one can see the fake horse bobbing around. (02:15:45)
Revealing mistake: When Jim is rescued from the snow (thrown over Reggie's shoulder), he moves his leg, although he is supposed to be unconscious.
Revealing mistake: When Rimbaud dies, as the nurse covers him with the white cloth, he blinks for a fraction of a second.