Marie Antoinette's mole switches sides and changes size between scenes.
Early in the film the guards whip Joseph, they then attempt to burn his eyes out. The iron is heated by a character out of uniform who passes it to a soldier. We see the scene from the back and the soldier moves the iron towards the child's eyes, a cut to the front has the non uniformed character moving the iron towards the child's eyes, a cut back shows the iron back in the hands of the uniformed soldier.
In the early scene between Cagliostro and Mesmer, Mesmer says that Cagliostro had never heard of "hypnotism" but was practicing it anyway. In fact, Mesmer himself never used the term "hypnotism." He called it "animal magnetism."
Right after the opening credits, the words "Paris, 1848" appear onscreen, quickly followed by a plaque on the wall reading "Alexander Dumas". But the author's name was as French as he was, and therefore the plaque on a Parisian wall ought to have read "Alexandre Dumas", and not its English translation. (His name is, however, properly spelled in the opening credits.)
In Balsamo's climactic escape attempt, Gitano passes him a pistol, seen very clearly as a small flintlock typical of the period. Balsamo then fires at least four shots from it without any time to reload. Repeating pistols not invented until the nineteenth century, and looked nothing like the gun used by Balsamo.