Continuity mistake: Caligula orders his chief gladiator to kill a prisoner so he can test the Robe's "powers". The gladiator stabs the prisoner with a dagger and straightens up. Look at the dagger and you'll see no blood on it.
Factual error: There is a shot of a large arena where gladiatorial combats take place. Arranged around the top of the arena are a number of classical statues, one of which is Michelangelo's statue of David circa 1505 AD, 1,465 years too early!