During the Siege of Breda, when Alatriste and his men are sent to undermine Flemish defenses, his second-in-command Sebastián is the last man to enter the cramped tunnels but appears right behind him anyway by the time the Spaniards are discovered.
During the white flag scene at Rocroi, the corpses around the Spanish soldiers move between takes.
During the opening of the Battle of Rocroi, the matchlocks muskets are firing without the serpentine or "hammer" holding the match moving. To fire a matchlock the burning end of the cord/match must swing down to the priming pan by the side of the matchlock.
In between shots during the Battle of Rocroi sequence, Captain Alatriste's musket changes (the muzzle is of different design).
During the Battle of Rocroi, cannonballs are hitting in the middle of the Spanish formation. The standard practice with cannons is to aim ahead of an enemy formation, that the cannonball may bounce through the front rank and cause as much damage as possible. They would seldom if ever aim for the center of a formation.
The calvarymen charging at the Spanish formation fire their pistols into the air, which does nothing except waste ammunition. They are more likely to be firing at the Spanish.
During the battle of Rocroi, a Frenchman can be heard ordering a cannon to fire, followed by the cannon firing. Cannons are situated well away from enemy lines, and thus wouldn't be in earshot of anyone shouting orders.
At the Battle of Rocroi, the cavalry soldiers only have handguns (that they wastefully fire into the air). Cavalry soldiers are armed with either swords or lances as well as handguns.
When Iñigo arrives at the beach after being freed from the galleys, a really tall modern building can be seen at the left part of the screen.
One of the Spanish soldiers is seen smoking a pipe with a vulcanite (para-caoutchouk) mouthpiece. Vulcanite was only available after the late 1800's.
Inigo is told on the galley that he has been pardoned by the King. However, since the galley was under-way at the time, the message cannot have reached the ship.
After their second encounter in the movie, Alatriste's nemesis and supposedly-skilled swordsman Gualterio Malatesta misses the mount of his scabbard and accidentally shoves his blade into a gap of his sword belt's fixings. The empty scabbard can be seen wobbling around while he's retreating.