Continuity mistake: During the battle at the convent, one of the Cardinal's guards takes aim with his matchlock musket, is shot by Portos, and falls - along with his gun - into a hay wagon. But if you watch the falling scene closely, it is evident that only the man ends up in the hay; the gun should have fallen some place to the left of the wagon.
Visible crew/equipment: When the musketeers are in the outer fortress having breakfast a bomb (like a hand grenade) gets stuck to a piece of bread. D'Artagnan throws the bread out the window and the camera zooms in as the bomb explodes. The explosion clearly begins inside the loaf several inches from where the bomb is stuck to the bread.
MiLady DeWinter: What do you want of me? There's nothing between us.
Athos: Thank God! But there might be.
Porthos: You know, it strikes me that we would be better employed wringing Milady's pretty neck than shooting these poor devils of Protestants. I mean, what are we killing them for? Because they sing psalms in French and we sing them in Latin?
Aramis: Porthos, have you no education? What do you think religious wars are all about?