Other mistake: When the burial of D'Artagnan parents begins, there is a shot from below, showing a beautiful view of the top of some trees, and the sky. There are 2 jet contrails visible.
Other mistake: Right before Febre kills the man who let D'artagnan escape, you can see a road sign made of metal in the background. They didn't make road signs like that back in that time. (01:07:05)