Other mistake: In the party/orgy scene where Aetius is showing off Rome to Attila, one can see a woman with her back to us reveal herself to a man and woman. Despite her veil, one can see her thong underwear. This type of underwear is a 20th century innovation.
Replacements - S1-E4
Other mistake: When Heffron is taking his throw at darts, he states that he needs a "double seven". When he takes the throw, the dart goes into the treble eight (an earlier close-up confirms that the board has the standard Gamlin configuration), yet, from the reaction he gets, he's made the throw he was going for. (00:04:40)