(at around 19 mins) In the bar scene during V's takeover of the television station, the cigarette of the long-haired man is almost burned to the filter. In the next shot of the bar, the cigarette is longer. In a third shot, it is back to the original length.
(at around 35 mins) In the moment when V is fencing against the displayed armor the sword of the armor falls twice to the floor.
(at around 3 mins) While Evey is preparing for her night with Dietrich, when she stands up to put her dress on, she takes the clips out of her hair that were holding her bangs back. In the next shot, her bangs are pulled back again, and alternate between being pulled back and let down, until they are left forward, when she turns off the TV.
(at around 1h 2 mins) Before the inspector starts talking to Dominic about St. Mary's and Three Waters, the inspector places an "anti-listening" device on a stack of CDs. In a later shot, the device is sitting directly on the desk and not on the stack of CDs.
(at around 1h 55 mins) In the fighting scene with Creedy's men near the end, in one shot, the last of the bodyguards loads his gun and points it at V. The camera then moves to V who throws his knife at the guard. In the next frame, when the knife hits him in the forehead, the guard's gun is down.
(at around 1 min) Guy Fawkes was not intercepted putting the gunpowder into position as shown in the movie. Fawkes was arrested by Sir Thomas Knyvet leaving the cellar underneath the House of Lords, having already lit a slow-burning fuse.
(at around 21 mins) The plot of Guy Fawkes was not a bid for freedom, as implied in the movie. The aim of the Gunpowder Plot was to create chaos among the English populace as part of a long-term plan to restore Catholic rule in the UK.
(at around 2 mins) In the opening sequence, Guy Fawkes is hanged with his hands tied behind him, dropping with the rope breaking his neck. In 17th century England, the sentence for treason was to be drawn, hanged and quartered. As quoted in Wikipedia - "Immediately before his execution on 31 January, Fawkes fell from the scaffold where he was to be hanged and broke his neck, thus avoiding the agony of the mutilation that followed." Although he was still quartered after death.
(at around 1h 55 mins) In a revolver, fired casings do not merely drop out by tilting the revolver backwards as Creedy does. The cases swell slightly upon firing and as such, they cling to the chamber walls. To remove them, the ejector rod needs to be pushed at the front of the cylinder which in turn pushes a star-shaped plate at the rear of the cylinder. The plate then pushes the casings out by their rims so that the spent shells can be removed. At no time does Creedy ever push the ejector rod.
The explosives were carried to the Houses of Parliament by an underground train, the explosion thus caused will not look like what has been shown on the film. i.e. the windows will not explode in a serial order and it will not cause Big Ben to explode.
The newspaper articles throughout the film include several headlines. Under these headlines, the articles are repeating text with virtually no correspondence with the headlines. This is possibly a tribute/reference to the original comic book, as that medium frequently uses repeating text to "represent" newspaper articles.
(at around 2h) At the very end while everyone is taking their mask off Gordon (Stephen Fry) makes a cameo appearance even though his character was announced as dead earlier in the film.
However, if you watch closely you will see every person who Evey is aware of being a victim of Sutler's regime in the crowd, implying that this was an intentional decision by the filmmakers to underscore the statement that Evey has just made to Finch about who V is (the victims of the regime).
(at around 16 mins) While the two detectives are driving to the TV station, the speedometer in their car is on zero, while their car is visibly moving.
For a few brief moments, Hugo Weaving's unscathed wrist and throat peek out through V's uniform. V is supposed to be completely covered in horrific burns.
(at around 7 mins) When 'V' first meets Evey he cutting his "V" sign on a wall poster. He made cuts with intersection over letter "O". However, in next scenes the shape of that cuts change several times, from having not any intersection until perfect "V" cut.
(at around 2h) The London Underground 'Roundel' sign on the pavement outside Westminster Underground Station is visible briefly in the closing scenes outside the Palace of Westminster still up and illuminated despite the Underground in the story having been closed several years earlier.
(at around 36 mins) When V goes to rip off the suit of armor's helm, while play-fighting, you can briefly see the metal rod that pops up to take off the head.
(at around 1h 26 mins) When V says that he has never danced to any of the songs on his Jukebox, a song is playing in the background, yet there is no record seen playing on the jukebox. This occurs even after V presses the button to activate the song.
(at around 1h) At Inspector Finch's home, in one shot, bottom left, we see a German-style mains outlet (where the studio scenes were filmed), not the 13 Amp UK socket it should have.
The computer monitors are Dell which is an American company. It can be inferred from the film that most would not want anything to do with America.
(at around 28 mins) When Evey first wakes up in V's home, he says to her, "You said they were looking for you." She never says that. She was unconscious before they left the TV network building and she first woke up just before this conversation.
(at around 1 min) The aim of the Gunpowder Plot was actually a scheme to blow up the House of Lords, not the House of Commons (commonly referred to as Parliament).
(at around 1h 30 mins) The files on Percy, Keyes and Rookwood, the three 'black-baggers' who either died or vanished the day after the St. Mary's attack, places the attack in 2014. However, Bishop Lilliman's tax records (at around 44 mins) show the events at Larkhill Camp, which predate the attack, occurred in 2016.
(at around 2h) When Finch tells Evey to back away from the lever towards the end of the film he pronounces the word in the American fashion (lev ver). In Britain the emphasis is is on the 'e' (lee ver). As the film depicts a Britain antagonistic to all things American it is doubtful that Finch would pronounce the word in this fashion.
(at around 13 mins) A female BTN newsreader in a red jacket is describing the demolition of the Old Bailey. She says that although the demolition went according to plan, the "fireworks and music were definitely not on the schedule." She pronounces 'schedule' in the American way, i.e. "sked-ule" instead of in the British way, which would be "shed-ule."
(at around 16 mins) When Finch is looking at the police record of Evey in the car, in the first part of Criminal History, it says Juvenile Reclamation Reclamation Project, when it should say Juvenile Reclamation Project.