Quo Vadis (1951)

Revealing mistake: 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.

Emperor Nero: Poppaea, one woman shouldn't judge another. She hasn't the glands for it. Ha-ha.

Trivia: Galva, the general who claims the emperor's throne after Nero's death, didn't last long as an emperor himself: just four months later, he was murdered by his Pretorian Guardsmen.