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.
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.
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 show the events at Larkhill Camp, which predate the attack, occurred in 2018.
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.
When 'V' first meets Evey he scratches his mark on a marble. During scratching it is seen that there is an intersection in the bottom part of letter V. But in the next scene it is seen that the two lines just meet not intersect one another.
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.
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.
During V's speech in the BTN, the first shot of Finch (right before he says "bloody hell...") shows him standing in an office. The next shot of Finch, during the same speech shows him standing in front of the BTN and no longer in the office, with no emotion on his face from ever having moved.
When the two detectives are examining Prothero's military deployment history (at around 57 minutes), Dominic pulls a binder out from a shelf, then sets it down with the spine of the binder facing to the right when he comes toward the camera to look at the computer screen with Finch. The shot cuts to the computer screen for a moment, then when it cuts back to the two detectives, the same binder can be seen in the background with it's spine now facing towards the left.
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.
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.
The original Guy Fawkes (of the gunpowder plot) was depicted dangling while hanging, kicking about until he finally succumbed. In historical reality he was sentenced to be "hanged, drawn, and quartered". He jumped when he was hanged so that he would die instantly to avoid the horrific "quartering" part of the then-usual English punishment for treason.
In the opening sequence, Guy Fawkes is hanged with his hands tied behind him, dropping with the rope breaking his neck. This might have happened in the USA in the 19th or 20th century. In 17th century England, the sentence for treason was to be hanged, drawn and quartered (see Braveheart). Guy actually killed himself by leaping from the scaffold. As quoted in Wikipedia - "Immediately before his execution on 31 January, Fawkes jumped from the scaffold where he was to be hanged and broke his neck, thus avoiding the agony of the mutilation that followed."
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 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.
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.
The goof items below may give away important plot points.
When Mr. Creedy is firing his revolver at V after his men have been killed, the camera, positioned presumably inside the revolver behind the cylinder, clearly shows two empty chambers out of six total, but Creedy eventually fires 5 shots. During this same sequence one of the empty chambers lines up with the barrel, but Creedy's gun somehow fires. Also, when the chambers are being viewed from behind the revolver, one or more seem empty. Even if a round had been fired, the shell casing would still be there.
During the final battle, when V throws one knife for the first time you can see in the lower-left corner that the knife is covered in blood. When the knife is seen again, close-up, it's clean. This happens again when he kills another Fingerman and throws him away and the knife in his left hand can briefly be seen with bloodstains on it and is, in the next shot, clean again.
When Finch is looking at the employees of Larkhill, the address for Bishop Lilliman reveals that he works and lives at St. Paul's Cathedral. However, when V enters through the window to kill him, the towers of Westminster Abby are seen as part of the building he scales
(at around 2 hours) The mob wearing their Guy Fawkes outfits are making their way to the square to witness the Houses of Parliament being destroyed and assemble on Bridge Street, directly below the Clock Tower ("Big Ben"). When the bomb is detonated, the entire building explodes, including the Clock Tower, sending bits of debris flying, which would kill the very crowd that had gathered to witness the spectacle.
When Evie is being processed (at around 1h 08 mins) the person shaving her head is wearing gloves you can partially see through. It is quite clear that the person shaving her head has normal looking hands, and not burnt hands like when V cooks her breakfast.
Right after all the soldiers have emptied their guns into V near the end of the film, he retaliates by throwing two of his knives at two of the soldiers. When he throws them, they are rotating at different speeds, but they impact at the same time in the same place on each soldier. You can see the knife farthest from the viewer stop spinning right before impact and slide into the soldier, even though it should have hit him at an angle.