Revealing mistake: When D'Artagnan and Rochefort are sword fighting on the frozen lake, both fall through the ice in separate places. As each one flails about in the water, it's obvious that the broken pieces of ice are slabs of Styrofoam. It also appears that what is supposed to be snow covering the ice is ground up Styrofoam, too chunky to be real snow.
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.
