Revealing mistake: In the first cathedral, the Bishop raises his hand before he extends it to Jocelyn and we see the underside of his ring. It is obvious that his ring is a costume ring because it's not closed in the back. This is confirmed on the DVD commentary. (00:23:35)
Revealing mistake: After William Thatcher loses agains Count Adhemar, he wins a gold jouster figure for best "sword on foot". Right after that, he cuts it in two with an axe. You can see that the figure is already cut in two, and that William almost doesn't touch the figure. (00:50:40)
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: At the very very end of the movie when everything has faded away and the camera pans in a circle around William and Jocelyn as they kiss, you can see a zipper in the back of Jocelyn's dress. (02:07:30)
Revealing mistake: After a naked Chaucer tells William of his gambling problem, and Simon the Summoner says that he owes 10 gold florins, when Wat angrily grabs Chaucer to punch him we see that Chaucer is actually wearing some type of underwear in the close-up. (00:26:05)
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.
Answer: The reason he is so agitated has to do with the manner in which he won the tournament. If you'll recall, William states, "I'll not be champion until I beat Adamar." Adamar had forefeited beforehand, (not wanting to joust against royalty) therefore not giving William a true victory. After his half won victory, Jocelyn's (Shannon Sossamon's character) inane chatter just rubbed him the wrong way. It had nothing to do with what she was saying...if anyone had spoken to him he would have reacted in the same way.