Continuity mistake: In the scene where William is having a discussion with Jocelyn in the stables, look at the top left corner of his breastplate. Between shots it changes from being tucked under his shoulderplate to being on top of it.
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.
Continuity mistake: During the last match between William and Rufus the sun changes between shots.
Continuity mistake: At the end of the film, when Heath Ledger has defeated Rufus Sewell, the amount of wood pieces in the air changes between shots.
Answer: Kate, Roland, Wat and Chaucer having a farting contest.
Grumpy Scot