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.
Continuity mistake: When Robin reaches the cliffs after escaping from prison, he jumps out of the boat and starts kissing the ground. He then reaches out with his right hand for help up, but when the camera switches, he's being lifted by his left.
Factual error: Aside from the earlier mentioned mistake of them apparently walking to Nottingham in less than a day, if Azeem was facing Mecca to pray, they set off in a southerly direction anyway.
Continuity mistake: When Robin heroically swings across to Fanny and the child in the Sherwood Forest battle scene: He swings straight across, with the rope's focal point fixed between himself and Fanny. Then he gives the rope to Fanny to swing across to John. This time the focal point is between her and John. Someone somehow managed to "move" the rope in order to move the plot along. That was jolly nice of them wasn't it?
Factual error: When Azeem goes to pray he starts by kneeling on the ground and soon afterwards he bows down. The Muslim prayer sequence starts by standing up for quite some time then one bow, up again followed by two kneeling bows then up again. Also, Muslims do not put their palms together for prayer as Azeem does, rather their hands are either at their side or crossed.
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.
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.
Continuity mistake: When Azeem first blows something up, the Friar is next to him and Will, Robin, John and Bull are seen behind them around the model of the castle. However when it goes to the next shot there's only Will and Robin around the model. John and Bull walk in a few seconds later.
Continuity mistake: When Robin goes to the Church, the camera pans down from an enormously high vaulted ceiling in a vast cathedral. A few moments later, Robin and the Sheriff come running out of the Church and it's a small little stone shack.
Continuity mistake: At the execution scene where Will Scarlet is about to be beheaded, Robin picks up a burning arrow to shoot the executioner. When the arrow is first picked up from a dead body, the flame is half-way up the arrow, but when Robin fires it, the flame is (conveniently) at the arrow's tip.
Continuity mistake: When Robin first goes to visit Marian, he enters the house with a roll on his back (over the shoulder). It's there when he enters and the maid tells him to 'stand right here'. A moment later "Marian" appears, and the roll is gone, it's not on his back, or even in a wide shot showing he took it off. It never appears again.
Continuity mistake: Throughout most of the film, Robin wears brown boots with metal studs around the calf. When he is fighting the Sheriff of Nottingham at the end, he is suddenly wearing greyish boots, with cross-garters. Since you see his brown boots just before he enters the castle - when he kneels down near the dung they're especially visible - it is rather ludicrous that he would suddenly change them, given that he is rushing around trying to save the other outlaws, and then Marian.
Continuity mistake: When Marian and Duncan leave the camp by boat, Marian's lady-in-waiting, Sarah, is nowhere to be seen. Surely she should have left with Marian, as she is with Marian later in the chapel.
Revealing mistake: When Robin gets attacked by Marian, Robin pushes her towards a stone wall several times, and the wall moves from the impacts.
Factual error: Think of the '100 billion dollars' scene in Austin Powers 2, where the President laughs because that kind of money doesn't even exist. You now have an idea how ridiculous it is that a twelfth century outlaw stealing tax money and robbing travellers could get anything like '4 million' as the scribe mentions. King Richard's ransom was only something like 100,000.
Continuity mistake: This mistake makes sense only if you've seen the extended edition. In a deleted scene, it is understood that the sheriff cuts out his scribe's tongue. In a later deleted scene, the scribe has to write messages on a chalkboard to communicate. Later still, at the end when Robin and company invade the castle, Robin asks the same scribe where the sheriff is and he verbally tells him, "Up the stairs," or something similar. Maybe he could speak a little, but he wouldn't have been able to pronounce the letter 'T'.
Continuity mistake: When Robin is challenged by John Little after attempting to cross the river watch Johns staff. The scene cuts between shots from in front of and behind John. All shots from in front of him show him leaning closely on the staff, all shots from behind show him holding it to side and not leaning on it.
Continuity mistake: When Robin is talking to the Bishop about his Father, they are in a small chamber in the Church. Later on in the film, when the castle is being stormed, Friar Tuck enters this room and throws the Bishop out of the window. Only this time the exact same room is in the castle, not the Church.
Continuity mistake: Guy of Gisbourne's men come upon two of Robin Hood's men in the forest, trying to pull a log onto the path. The men see their enemies and run away, leaving the log sticking out onto the path. But the log suddenly disappears when Gisbourne's men go after them, and never again reappears.
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.
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.