Continuity mistake: D'Artagnan's three best friends who have known him for over twenty years pronounce his name three different ways. Only the stupid one, Porthos (Gerard Depardieu) gets it right. "Athos" is also pronounced several different ways: either a hard or soft initial 'A,' and with the 'h' either pronounced or silent. Sometimes the name is pronounced different ways in different scenes by the same character.
Continuity mistake: Close to the end of the film, when D'Artagnan is helping everyone escape through a back door of the Bastille, he says "Don't stop until you reach the river" and we see the door opening with all the characters in a particular order. I believe Athos is on the left of Phillippe. In the next shot, we see the same door opening from the outside and now the characters are lined up to exit in a completely different order. Athos is now on the right of Phillippe.
Continuity mistake: When Phillipe is having his mask removed after being rescued from the Bastille, he touches both sides of his face with his hands in a close shot of him. But when it goes to a wider shot of him and the rest of the men, when he turns to look at the water bin, his hands are no longer touching his face. There wasn't time to bring them back down.
Continuity mistake: When Louis hits Phillippe when they are in the Palace, it seems he hits the right side of his face, but Phillippe falls holding the left side.