Visible crew/equipment: In the very first scene, the boom mic is very obvious at the top of the screen.
Factual error: 6 years in a mask and he's able to keep that lovely babyface with just a little bit of grime, which easily comes off? Of course it would have wrecked the whole story but still...
Continuity mistake: The mask that is removed from Leo has 3 parts - a face mask which is held onto Leo's head, with a 2 piece hinged cage that locks in the back. In the flashback, where they show Leo being put in the mask, it is only 2 pieces. The face part is fused to the hinged cage section.
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.
Audio problem: At the mask-ball, when Louis is seeing iron masks everywhere and rushes out, the music suddenly stops, but in the background we can see the violinists still playing (moving their bows) - we don't hear any sound.
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: Towards the end of the movie, (when he gets in the shallow boat with the Musketeers and Louis), Phillipe seems to have red bows on his court boots, yet when he was at the masked ball the bows were gone.
Continuity mistake: When Phillipe stands up to look in the water bin after he had his mask removed, he places only his left hand on the bin's edge. But in the next shot of him looking in the water, his right hand is on the edge instead of the left.
Continuity mistake: Why is the same red rose used throughout the film? If it was real over the amount of time in the film the rose would be long dead. Look at the branching of the rose and you will see what I mean.
Factual error: The film is set in the year 1662. Louis is supposed to be in the middle of a war against the Dutch. This is actually not correct. The war against the Dutch didn't begin until 1672.
Other mistake: When the Musketeers are charging the firing squad at the end, they only have about 20 yards to run which they should cover in just a couple seconds, but somehow it takes them much longer to reach the other men. This is especially noticeable since they're running at full speed. (This has nothing to do with editing or slow-motion, as most of the shots are shown in real time, at regular speed.)
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.
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.