During the fight at the burning building Porthos attempts to escape by performing a 'death slide'. When he begins the slide he clear had bare hands, when he finishes the slide (just before he sets himself on fire) we can see he is clearly wearing gloves.
After the bedroom fight between D'Artagnan and Milady, in the café scene, when D'Artagnan stands up and accidentally drops a piece of jewelry on the table, Athos grabs D'Artagnan's hand with his right hand while his left hand is holding a pipe in his mouth. In the next scene, Athos is holding his left hand over D'Artagnan's, while his right hand is holding the pipe.
when D'Artagnan is struggling in the ice he picks up his sword in an effort to defend himself from an approaching attacker. In a long shot he plainly las the sword yet in the close-up he has yet to pick it up.
In their race to save Constance at the Convent, several of the Musketeers leave the inn on horses which have had their saddles removed ... bare back. Yet when they finally arrive at the Convent and dismount, it is clearly evident each horse is fully tacked.
After the Musketeers rescue Rochefort, he puts on his hat twice between shots.
After Felton leaves Milady's cell in the Tower of London, he stops to take a drink of water from a metal tap/facet, these types of taps weren't invented for at least another two hundred years...
In the swordfight on the frozen river, the lumps of broken ice float high in the water like Styrofoam. Real ice would be 9/10 below the surface and water would slosh over the lumps as they were moved around.
When the bomb attached to the bread is thrown back at La Rochelle, clearly it is the bread which explodes. If the frame is stopped, the fuse is lit near it top and the bomb is still clearly intact as the bread explodes.
When Felton purchases a knife to kill the Duke, he hands the vendor a coin and walks away. The visuals then fail to match the dialog. The vendor tosses the coin into a bowl, then he takes another out, and then walks away. The dialog says, "...tuppence, don't you want your tuppence sir?" The vendor never offers change to Felton, and before the sentence is complete, the vendor has already walked away.
The English troops being reviewed by Buckingham are carrying Union Flags. Although that flag did exist at this date, it was not used by the Army until the Act of Union (which brought England and Scotland together as one State) roughly seventy years later - they should still have been carrying flags bearing only the Cross of St George.
At the time movie takes place (early 1600s) firearms were either matchlocks or wheellocks, or flintlocks. So the musketeers and their rivals using matchlock muskets is correct, but Lady de Winter's percussion pistol wouldn't be invented until the 19th century.. The musketeers are also shown using persussion pistols at thee siege of La Rochelle, Rochefort has two visible inside his carriage, and Porthos fires one during the fight at the convent. These all appear to be flintlock pistols, as the frizzen can be seen, and pre-dates their invention by about 50 years.If they had been using pistols at all, these would have been horse pistols, mid-way between the small pistols shown, and the flintlock muskets, which were in use atr this time.
Although the artillery pieces used by the rebel (Protestant) garrison at La Rochelle, hidden behind gabions, appear to be more authentic, the loyalist (Catholic) besieging forces appear to have batteries of guns that could not possibly date from 1628, when the garrison surrendered, as these have barrels, and indeed wheels, that are far too slender, and would appear to be replicas of pieces dating from the early 19th century. Both sides also fire projectiles that explode on contact, which wasn't the case. At this point in history, artillery pieces fires solid shot only, not even canister.