Revealing: In the scene when the Christians are being fed to the lions, we can see that when a lion "attacks" one of the men, the man is actually smiling and visibly playing with that wild animal.

Other: In the scene where one of the Christians is being crucified and burnt, we can see that just after the man's "death" the body is substituted by a plastic doll covered in red paint.

Continuity: When the giant is fighting the bull, in the long shots the bulls horns are white, but in the closeups, they're black.

Deliberate "mistake": Wooden crosses are shown in several scenes, hanging on a wall. Crosses did not come into use as a Christian symbol until 450 A.D. (about 400 years after the film is set).

Factual error: The Pretorian Guard was not as loyal to the end as shown in the movie. Actually, they were bribed into rebelling against Nero.

Visible crew/equipment: An absolute cracker from an epic movie - in one of the huge crowd scenes when they all go through the huge gates, you can clearly see a wee man in blue overalls eating a sandwich, he quickly realises there is a shoot on and tries to blend in with the crowd and enters through the gates.