Continuity mistake: When Robin, Azeem, and Duncan visit Marian's house for the first time, it's dreary and overcast out. But when Robin steps inside, there's suddenly bright sunlight coming through the windows. It then periodically cuts back to outside, where it's still overcast.
Other mistake: Michael McShane's name is spelled wrong ("Micheal") in the opening credits.
Continuity mistake: When Robin arrives on the beach at Dover he drops his 'roll' then falls onto the sand, rolling in the shallow water. The shot from behind his head shows the roll but when the camera moves to the front the roll has gone.
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.
Visible crew/equipment: Following the river duel the outlaws, Robin and his men sit around a campfire in the woods. There is a high-angle shot looking down on the men, also showing Little John perched in a tree. Look behind the tree, through the branches, and you can see a piece of camera tracking on the ground.
Continuity mistake: When Robin is talking to the Bishop in his private chambers, they're sitting by the window. They're both lit on the side of their faces that are near the window, but for some reason in the Bishop's closeups, he has harsh white light on the other side of his face as well. Not only is there no source for this light, but in the wide shot that side of his face isn't lit.
Factual error: Marian tells her messenger to take a letter to the King, who is in France. Problem is, we were enemies with France at that time. The King wasn't even there, he was on his way home from imprisonment having been captured on his return from the Crusades.
Audio problem: When Marian is attacking Robin inside the house, and Azeem is trying to break down the door from the outside, Duncan says "Point me towards danger, Azeem. I'm ready." But his mouth doesn't move at all.
Continuity mistake: Just after Will is tied to the barrel to be executed and the drums start rolling, there's a wide shot of the whole gallows, including the barrel, but Will is missing.
Continuity mistake: When the outlaws are being attacked in the forest, one man is shot by a flaming arrow in the back. But when he falls off his platform onto the ground, the arrow is suddenly in his stomach.
Factual error: This pertains to all Robin Hood movies. Robin's back quiver was an American Indian innovation, unknown in Europe until the Age of Discovery. Medieval archers used belt or shoulder quivers.
Continuity mistake: When John starts to cross the burning bridge to rescue his wife and baby, he steps onto the bridge and almost into the fire, which is just inches from him. But when the bridge breaks, the fire has suddenly moved several feet below him (so it doesn't burn him up as he climbs to safety, I presume.).
Continuity mistake: As Marion begins to distract Robin from aiming the arrow you will notice that at the wide angle shot she picks up a brown fletched arrow. As the shot tightens it is a white fletched arrow.
Factual error: In the religious service there are some incredible mistakes that are really obvious to a historian whose speciality is medieval church architecture. The scene is filmed in St Bartholemew The Great Church in London, which was founded in 1123 and built during Henry I's reign: thus it would have been standing by the reign of Richard The Lionheart, when the film is set. However, several seventeenth century memorials can be seen on the walls of the church, and even a modern wooden hymn board. Some of the upper windows of the church were added in the fifteenth century: we see these on several occasions. The glass in the windows is obviously modern, and while the interior walls of medieval churches were elaborately painted, the walls and stonework are plain and bare. (Admittedly it might have been rather expensive to install coloured medieval stained glass and paint the interior walls, so perhaps we can let that go.) There is a later scene in St. Bartholemew the Great in which a modern altar, candlesticks and metalwork can be seen.
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.