When first talking to Cassia in the balcony, Senator Corvus refers to Rome as "The Eternal City". Yet, the nickname was not coined by the Romans but by the Christians, much later, when Rome was already the seat of the Papacy.
By 79 A.D. when the movie takes place, the color purple had become reserved for the emperor's personal use. The emperor Nero had earlier ordered the execution of people who dared to wear purple. Corvus wouldn't have worn purple. Nor given that purple dye was incredibly expensive, would he have been able to outfit his soldiers in that color.
When Cassia's father examines the damage to the amphitheater, he comments that it has stood for 100 years. Pompeii has the oldest amphitheater in the Roman world - it was built in 70 BC, so would have been almost 150 years old AD 79.
When Corvus puts out his arm with clenched fist, about to give the order to kill Milo and Atticus, he does so with his fist horizontal. In the next shot it is vertical. Then, in the following close-up shot, it is once again horizontal.
After Milo's sword is cut in half, Proculus has his sword right under Milo's chin. The camera shifts to one side, and the sword is a few inches away from his chin. When it goes back, it's under the chin again.
When Atticus is about to die, and you see his face from straight ahead, Proculus' body behind him is facing Atticus' right side. When there is a wide shot from the side, Proculus' body is now facing to Atticus' left, the way he fell when he died.
When Milo fights with Atticus with the wooden sword in the arena, and Milo takes the sword from Atticus, Atticus take one sword from a man standing around. First it is visible that this gladiator has no helmet. Then when Atticus takes the sword the man has a helmet, but then he kicks the gladiator who fell down to the ground without any helmet. Also it is visible that Atticus in the closeup goes to different gladiators compared to the gladiators stands behind Atticus before.
Pompeii is not located between Mount Vesuvius and the coast. The volcano is northwest of the city, while at that time, the coast was just to the west. The location of Pompeii in the film is more consistent with Herculaneum.
The ancient town of Pompeii was not directly on the ocean. Although the coast was nearer to it in ancient times than today, its walls (which defined the city) ended about a half mile away. However, the nearby separate town of Herculaneum, also devastated during the eruption, was on the ocean.
Pompeii's amphitheater is located in the south-east corner of the town. It is shown in its correct location in the opening sequence, but has moved into the town center when Milo and Cassia are riding in the hills.
When Cassia and Ariadne when traveling from Rome to Pompeii, as they first see Pompeii, the Mediterranean Sea is on their left suggesting that they are traveling north toward the city with Mt Vesuvius in the background. Since Rome is north of Pompeii, Mt Vesuvius should have been behind or to their left as they traveled south along the Mediterranean coast, which should have been on their right side as they approached the city.
Pompeii is a bubbling Volcano throughout. The reason Vesuvius caught so many people off guard is that they had no Idea it was a volcano. Before the eruption rather than looking like two peaks it had the appearance of a table top mountain. The void between the two peaks was blown out during the eruption.Therefore the film depicts a post eruption Vesuvius prior to the eruption.
The "stone statues" of victims were not made of stone or ash, as implied by their appearance in the movie. They were actually made of plaster in modern times, from material injected into cavities remaining from bodies left in the ash.
Gladiators were of the lowest class in Roman society (known as 'infamii'). Though they held prestige when in the arena, they were hated and despised in day-to-day life. Proculus, as a tribune, would never had lowered his social status to become a gladiator (Corvus announces him as "champion of Rome"),or, if he was a gladiator to begin with, he never would have climbed the social ladder to become a man with considerable political authority, Either way a senator like Corvus would never have associated himself with Proculus.
The phrase "We who are about to die salute you" was only spoken by criminals condemned to die in the arena, and would never had been said by professional gladiators. Likewise, arena combats recreating historical or mythical battles would only have involved criminals, not gladiators.
Pompeii suffered a serious earthquake in AD 62, resulting in damage to most of the town and causing numerous inhabitants to leave. As a result of the earthquake, gladiators were held in temporary barracks in the portico of the theatre, not in the amphitheater itself.
The festival going on at the time of the eruption of Vesuvius is portrayed as the Saturnalia, which is held in December (it was pagan Christmas). Vesuvius erupted on the 24th of August, a day after the Vulcanalia (a festival for the god of fire, Vulcan) ended.
Vesuvius did not spew lava in the 79 ce eruption. The film correctly shows a massive pyroclastic flow (a mix of very hot ash, rock and gases) which travels at very fast speeds up to 700 kph. This is what destroyed and blanketed Pompeii.
At 1hour 17 minutes in the boat scene where Graecus is trying to flee with the others, he is knocked off his feet when the boat is hit from flying debris and is sinking the water washes over him and must knock off a wig that he must wear throughout the film (apparently rich men wore wigs as well as women in Roman times) revealing a modern hairstyle.
The goof items below may give away important plot points.
Towards end of movie, Corvus has shackled Cassia to the chariot with her right arm. Right before she gets loose from the shackle, there is one shot where she is shackled with her left arm. In the next shot, it is back on the right arm.
From the topography, Cassia's villa is the Villa of Mysteries, just outside the town of Pompeii. It is possible to visit the villa today therefore it cannot have fallen into the sea, as depicted in the film.
The pyroclastic surge that engulfed Pompeii and the surrounding areas was probably moving at anywhere between 90-130 m/s (200-290 mph). Suffice it to say, none of the dramatics that took place at the movie's end (Atticus's declaration of freedom, Milo and Cassia's final kiss) would have happened--they literally would not have seen it coming.