Plot hole: The Bishop in the film is performing the duties you would expect of someone in his position (giving mass, hearing confessions, performing weddings etc.) He has his own private chambers in the cathedral, refers to Robin as 'the boy I knew' and talks about hearing his Father's confession four months earlier; so he's been around for a while. The problem is the credits refer to him as the 'Bishop of Hereford'. No explanation is ever given for why the Bishop of a city 100 miles away is living and working in Nottingham rather than looking after his own diocese; or why the Bishop of Nottingham isn't around to look after his. (The Bishop of Hereford was an enemy of Robin Hood in the original ballads, and it's likely the filmmakers just gave that name to the Bishop in the film due to its familiarity, without thinking about the plot hole this creates).
Factual error: Robin and Azeem land at the white cliffs and Robin says something like, 'Tonight we will dine with my father'. Those white cliffs are the Seven Sisters in East Sussex, about 200 miles from Nottingham. It would probably take about two weeks to walk it. They also reach Hadrian's Wall in the next scene, which is 200 miles further north than Nottingham.
Continuity mistake: When Robin is burying his father, he cuts his hand saying, "I swear it by my own blood" and that he will avenge his fathers murder. We see the blood dripping over his fingers. Suddenly the camera shot changes and Robin's hand has no blood seeping between the fingers.
Factual error: There is no reason why the executioner would have an axe just conveniently lying around on the scaffold to use on Will Scarlet. All the criminals were sentenced to hang, he wasn't expecting to behead anybody. (For those thinking this might just be a contingency - beheading as a punishment was only ever used on members of the nobility. It's not like the executioner or the Sheriff would have prepared this as a back up 'just in case' - public executions didn't work like that).
Factual error: When Marian is embroidering at home, she is singing to herself "Le chant des oiseaux", which Renaissance composer Clement Janequin didn't write until the 16th century.
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 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.
Continuity mistake: At the start when Azeem is tied up in the prison, at one point when he asks the Christian to free him, his hands are tied at the wrists, then apart tied to the wall, and then back to his wrist all in three quick takes. (00:05:15)
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?
Continuity mistake: When Robin reaches his homeland, after he kisses the ground, he rolls over in the water. His coat is covering him, then in the next scene, it isn't, then it is, then it isn't. (00:12:15)
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.
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.
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.
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.
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.