Revealing mistake: When the boat is supposedly surrounded by a swarm of flies, they're really just relatively few flies trapped between two panes of glass in front of the camera lens. Evidence: whenever the camera moves, the "swarm" moves with it against the background, and you can even see some of the insects walking on the surface of the glass.
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.