Visible crew/equipment: When they are moving the catapult, if you look above Robin and Azeem, you can see a crew member in the window above them. He is wearing all black, but you can tell he is trying to get out of the shot.
Visible crew/equipment: When the men are pushing the catapult to the gate look at the far left of the screen for a few seconds you can see a crew member in a baseball cap and then a large movie camera. There's also a twig palisade next to him where two more crew members are visible.
Other mistake: Michael McShane's name is spelled wrong ("Micheal") in the opening credits.
Answer: The white robes, reference to "the old ways", and pentagram across the map when the Sheriff meets the Barons suggests per-Christian Druidism; the upside down crucifix certainly implies Devil-worship. These two spiritual paths are, by nature, mutually exclusive. In short, a fictional pseudo-witchcraft invented for the film, yes.