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: 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).
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.
Factual error: The Sheriff of Nottingham seems to use timekeeping as we do today in the modern age of clocks. However, in the 1100's, when the film takes place, an accurate tell of time was only available during sun-up hours using a sun-dial that needed daily adjustment in order to stay accurate. So sundials would save this mistake, however, the use of torches in the scene as well as an additional source of cool light (the moon) suggests it was nighttime when 10:30 and 10:45 were mentioned. (01:05:30)
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.
Chosen answer: The masked men weren't part of a cult, they were the Sheriff's soldiers, the same ones that Robin meets when he first returns home. They are just in hoods and masks to appear intimidating while trying to kill the noblemen, like Locksley.
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