Nearly all of the characters have straight and perfect teeth. This would not be possible given that most dentistry of that era consisted of pulling teeth. Additionally, there are several scenes where it is clear that the actors have fillings which wouldn't been available until several centuries later.
The fighting styles shown resemble Asian martial arts more than European combatives of that era. It would have been at least 200 years before the Northern Europeans would trade with China and their arts became known in the West, It wasn't until after WWII in the 1940s that Westerners adopted their fighting styles en masse.
During the fight at the castle, D'Artagnan is fighting multiple guards while hanging from a rope. At one point, he slides down his own rope and is clearly seen dropping his sword. However, when he climbs back up, he resumes the fight with a sword in his hand.
Due to its granular nature, gunpowder of the period was highly susceptible to moisture. Once even a tiny bit of moisture got into any amount of gunpowder, it was very difficult to ignite, even impossible. Therefore it is highly unlikely that any of the muskets, pistols, or cannons used in the final battle would have fired.
The swords carried by the majority of the characters are rapiers and, as such, are thrusting rather than slashing swords due their relatively thin blades and narrow profile. There are multiple scenes were the sword is shown to kill opponents by slashing them across the chest which would have caused a lacerations, but not mortal wounds.
During the final ladder fight scene, D'Artagnan stabs Febre through the ladder after sliding all the way to the bottom of the ladder, but in the next shot as D'Artagnan walks away Febre's corpse is 8 feet farther up the ladder.
At the climax of the ladder fight Febre knocks D'Artagnan's sword out of his hand. When D'Artagnan is then knocked down the ladder he doesn't retrieve it. So it appears that he stabs Febre, who is sliding down the ladder, with his empty hand.