Factual error: Before the eruption, we see Mount Vesuvius with a crater. Mount Vesuvius did not have a crater prior to the eruption in AD 79. This can be seen in a fresco depicting Mount Vesuvius found in Pompeii.
Factual error: In a brief scene showing a map of Europe as it was in 79 AD, complete with Latin. Rome is displayed with the Flavian Amphitheater (Roman Colosseum), however, Pompeii was destroyed in 79 AD and the Colosseum did not open until 80 AD and was not finished to look as the Colosseum we know it and as shown on the map in the movie, until many years after that under the rule of Domitian, the brother of Titus.
Continuity mistake: In the scene after The Celt defeats the big Thracian in a fight at the slaves' "new home", Master Graecus comes to get Bellator's explanation about the fight. After Graecus leaves, Bellator says "I'll remember you, Celt." Before Bellator says that, The Celt's left collar is below his shoulder. Just after Bellator says that, the camera cuts to show The Celt and his left collar is now on his shoulder. (00:19:25)