Continuity mistake: In the scene where Chris O'Donnell and Kiefer Sutherland are riding in the forest together, they are both on white horses. Kiefer Sutherland's white horse gets killed, and he later rides into Calais on a brown horse. In a later scene when the four are riding together to gather all the Musketeers, there are sometimes three white horses and one brown horse, and sometimes there are four white horses.
Continuity mistake: When D'Artagnan meets Porthos he accidently spills wine on Porthos' sash who then gets up and is seen holding the sash between 2 fingers. The camera angle changes and he is now seen with the sash just laying in his palm and then he's seen again with the sash between his fingers when the angle changes again.
Continuity mistake: When D'Artagnan is riding away from the brothers in the beginning, he is seen riding the horse into woods from the wide camera angle, but when the angle changes to a close up, you see D'Artagnan riding along the edge of the woods for a few feet and then turning into the woods.
Continuity mistake: When the patrol goes into the house and finds Athos and Aramis sitting at the table, Aramis is cleaning his sword with a cloth up to the middle, and the rest of the sword is covered in blood. They converse a little, and when Rochefort steps forward (and Aramis says "Ah, hello again, Cyclops"), Aramis places his sword on the table, although now there is no blood on it at all. He had no time to clean it off, and even if he tried there is not sufficient blood on the cloth to suggest he did.
Continuity mistake: When Aramis is wooing the wench, her hair changes from shot to shot. When he first speaks to her, she has small wisps on her left side. When the shot changes to D'Artagnan's point of view, she has a lot more hair down on her left side. As she leans to Aramis to kiss him her hair is down on the left side, and the shot of her kissing him shows her hair behind her ear.
Continuity mistake: In the scene where D'Artagnan is being chased by Gerard after the duel for his sister's honor, as they approach the overpass stunt that ends with Gerard falling on his high-pitched butt, the swords at their waists disappear and reappear while changing positions on the horse.
Revealing mistake: In the fight scene right after the guards interrupt the duel, Porthos breaks his opponent's sword with a 3 bladed knife and then he throws a balled string which wraps around the guard's head. When the scene shifts to the guard's close up you can see the guard throw the wrap around ball himself.