Audio/video fuori sincrono: When Lygia calls out to Marcus during the burning of Rome, her lips do not move.
Continuity: When Nero holds out his arm to recognize Marcus' triumph, his arm is obviously straight. But, in the next shot, from above, his arm is bent.
Factual errors: Nero in fact died four years later in history than shown in the film. However, the events surrounding his death (citizens revolting, committing suicide with some outside help) are true to fact.
Factual errors: At the time of Nero, the Vatican was home to a circus, not an apocalyptic cesspit as portrayed in the film. Scholars believe that Peter was most likely martyred as side entertainment during a chariot race, or similar event.
Anacronismi: Chess was unknown in Europe until at least the seventh century, yet we see soldiers playing.
Revealing mistakes: When Nero plays his lyre as Rome burns, the seams and stitching of the night "sky" behind him are clearly visible.
Anacronismi: Construction did not begin on The Colosseum, where the climax takes place, until 70 A.D., 6 years after the movie is set.
Continuity: When the giant is wrestling the bull in the arena, note that in clinches in close-ups the bull's horns are almost entirely black, whereas in distant shots, the horns are mostly white.
Anacronismi: Audio anachronism: I seemed to hear at one of the scenes at the beginning, when Nero-Ustinov is entertaining guests at his palace, a sentence in French! He turns around to address one of the musicians, and says something like: Joue pour moi! (Play for me). This was an original soundtrack version showed in Spanish TV, on April 5th 2007. I don't know if this was a joke or an incredible anachronism, since French was derived from Latin many centuries afterward. Maybe a hint to Nero's snob-ism...
Factual errors: At the beginning two Generals are seen arriving at Rome by Chariot.The Roman Army of the 1st Centuary AD did not use chariots.
Factual errors: In one scene people are seen eating dinner at a table. Elite Romans ate reclining on couches.
Factual errors: A character asserts that the City of Tarsus is in Nothern Palastine, which Paul of Tarsus confirms. In fact it was in the Roman Province of Cilicia in modern-day Turkey.