Continuity mistake: In the scene just before the final attack on the castle, D'Artangan is surrounded by his fellow Musketeers. He gets off his horse and while doing so there is a tight close up where you see clearly that he is wearing his Musketeer outfit that he threw down on the floor in the previous scene. When he finishes talking to the horse (and he is back in his normal clothes) Porthos hands him the Musketeer uniform, which D'Artangan then puts on. (01:20:45)
Continuity mistake: At the beginning of the movie, D'Artagnan is shown as a child with the old man who raises him. Soon after it is 14 years later - the old man is wearing the exact same clothes and there are no changes to his appearance.
Factual error: In 17th century France a peasant would not be using metric measurements. Just before the final battle, D'Artangnan states the castle is 2 kilometers away. Metric was developed by French scientists AFTER the revolution.
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.
Continuity mistake: Young D'artangan speaks with an accent. His older self does not.
Continuity mistake: While the character Mena Savari plays is in the credits as Constance, she is referred to as Francesca many times throughtout the movie. In fact, I can't recall a single time she is referred to as Constance.
Continuity mistake: When D'Artagnan is fighting from the ropes on the side of the tower, there is a shot of his rope being cut and him falling but in the next shot we see him climbing the tower again as if it never happened.