During the fight between Athos and Rocheforte in the end battle scene, Rocheforte is wearing a sword sheath. Athos knocks him down, and when he rises to begin fighting D'Artagnian in the next shot, the sheath and belt are missing.
In the final fight between D'Artagnian and Rocheforte, D'Atagnian stabs Rocheforte on the left side of the row of buttons, five buttons down. We cut away to D'Artagnians face and in the next shot, the sword is on the right side of the row of buttons, three buttons down.
In the final fight scene between the musketeers and the cardinals guards, D'Artagnian is fighting the assassin. Porthos shoots the assassin with a crossbow from the ground and the assassin clutches the arrow in his chest and falls. As he falls, you can see that the arrow is gone.
In the chase scene between D'Artagnan and Gerard both riders have empty scabbards but when they both reach the tree and jump over it, their swords return.
As D'Artagnan is preparing to leap up onto the fallen tree during the initial chase scene in which he is being pursued by Girard and his brothers, his rapier is conspicuously absent from its scabbard.
The copyright date (in Roman numerals) is incorrect in the end credits (it is missing an 'M'). The film shows (c) MCXCIII (1193) when it should read (c) MCMXCIII (1993).
Anne of Austria is referred to as being from Austria. She was actually Spanish, a princess from the Austrias Dynasty, the Hapsburgs of Spain.
Porthos mocks Rochefort as having the name of "a smelly cheese". Not only are the character and the cheese from different regions with different names and entirely different pronunciations (Roquefort for the cheese, Rochefort for the character); but the cheese was given its denomination in 1925, almost three centuries after the events of the film.
Aramis is shown quoting Genesis (the first chapter in the Bible) while presumably reading from a Bible, which is open in the middle. Given Aramis' reputation and the subsequent action, it is possible that he was quoting from memory and merely had a book open in front of him to give the impression of piety.
When the "ugly thing" in the dungeons is killed by Porthos, two of the spikes on the wall can be seen bending in the close-up shot as he is pushed against them.
When Rochefort angrily slashes the three candles with one sword sweep, the remaining stumps of the candles are all of different heights.
In the final fight between D'Artagnan and Rochefort, after Rochefort has knocked D'Artagnan down the stairs, we cut to see him rolling. There are no stairs in this shot, he is obviously rolling down a ramp.
In the shot where D'Artagnan first sees the Queen and Constance riding ahead of the Queen's guard, a modern farm with aluminum silos can be seen in the background.
During the carriage scene, Charlie Sheen says, "End of the line, boys" - a railroad term that would not come into use for about another 200 years.
The crossbow bolt used to send the message to all the Musketeers is clearly a modern one, as it is a hollow tube with solid (probably plastic) fins. It should have been wooden, and the fins would be feathers.
Velcro fasteners on the Musketeers' over-shirts.
There is a ship in the harbor with a Plimsoll line. This was not invented until 1870 to stop ship owners from overloading ships.
In final fight scene, modern track lighting can be seen on the castle ceilings.
Obvious stunt doubles in the carriage chase scene.
When d'Artagnan knocks the assassin from the roof, the assassin's wire is visible during his fall.
When the assassin is shot off the wall you can see the pad he falls into for a quick second before he hits.
Aramis stated that it was 200 leagues to Calais or 600 miles. Actually it is only 180 miles from Paris to Calais.