There is a scene near the end where Romulus Augustus is wearing the Caesar's sword scabbard with the shoulder strap over his right shoulder (scabbard on his left hip) and in the next shot, it is hanging from his left shoulder and then in the next shot it is again on his right shoulder.
Romulus Augustulus was born on 460 A.D. At the time of the movie, he should have been a newborn baby.
The real Romulus Augustus was born more than two generations after the Romans abandoned Britain.
At the end of the movie, during the climactic battle, the infantry marches into battle holding a banner high and proud. Unfortunately, the banner is the Draco, which was a cavalry banner, not an infantry one.
The invention of stirrups did not reach the Roman Empire. This invention arrived only after the fall of their reign, around the 7th century. As such, the horsemen could not have used stirrups as seen in this film.
As they are climbing over the mountain pass, Romulus is riding on the back of Batiatus wearing a pair of very modern climbing boots.
The castles depicted are crenelated Norman castles, not present until a century after 1066.
As Romulus passes Caesar's guard in the coronation procession (at around 9 mins), Demetrius mouths the words "We're dead" (In the DVD commentary it is explained that "we took it out just because it was too soft to be heard over the noise of the procession and the music").
After Aurelius and Mira jump from the wall of the villa on Capri and swim to the boat, Demetrius grabs Aurelius's sword by the hilt with one hand, then grabs the blade across the supposedly sharp edge with the other hand.