Revealing mistake: If you watch the deleted scene "Chaucer's speech" on the DVD you can see that when William is in the stocks, Chaucer makes a nice speech about him and the crowd starts chanting "William, William...". Well in the movie it is made to look like they are angry with him but if you watch their mouths, they are mouthing the name William, but all you hear is yelling. (01:52:00)
Revealing mistake: In the joust montage sequence when William allows himself to lose repeatedly in order to prove his love: Just after the shot of the white flags going up, we see "William" get hit again but it reveals itself to be a mechanical double first in the way it folds over backwards in a sharp 90 degree angle (no human can bend that way), and it reveals that there isn't a person within because it breaks with the lower portion of the "body". Shot from another angle (the right) the front portion of the saddle blocks this from view.