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.
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: When Wallace rides into the village after his wife is killed, he hits the foot soldier with the mace-like weapon. When his horse is lanced, note that the lance is actually attached to the leather girdle on the horse, so, in effect, the animal is merely being pushed over.
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.
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.