Revealing mistake: When Robin crashes through the stained glass window to save Marian and fight the sheriff, the panes of glass flutter in the breeze, revealing it to be fake glass.
Revealing mistake: When Friar Tuck hits his head on an over-hanging branch whilst on his cart, it is really obvious his head comes into contact much earlier than the added sound effect and his eventual reaction would suggest. Plus the branch bends and flexes backwards an awful lot - it's not resistant enough to knock someone out as depicted.
Revealing mistake: When Robin gets attacked by Marian, Robin pushes her towards a stone wall several times, and the wall moves from the impacts.
Revealing mistake: When Robin Hood is fighting the sheriff's men under the tree, Robin throws a crossbow at one of the men. You can see the plexiglass, which is protecting the actor, get marked up and shake from the impact. (00:18:02)
Revealing mistake: The fallen tree from which Robin gives his inspirational speech about freedom has obviously had several branches removed with a chainsaw. It may be hard to see the marks of the blade on a copy of the video, but in the cinema they were pretty obvious.
Revealing mistake: This happens at the beginning of the battle with the Celts when Robin runs up the hill to fire a few arrows at them. When he kills the first one, you can clearly see he leans back on his horse as if struck by an arrow, before the arrow actually hits him in the chest.
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.