The width of the wall between the front outside door and the bay windows varies between shots. The wall width of the actual location shot is roughly five times smaller than the studio shot. This is noticeable throughout the entire film and is highlighted by the placement of the old leather chair and the size of the hallway.
(at around 53 mins) When Maurice Grosse speaks with the possessing spirit Bill, the spirit plays the Knock, Knock game. Grosse is recorded as clearly pausing for a second or so before saying in a flat tone with a falling inflection, "Very well, who's there?" When the recording is later played by Father Gordon to the Warrens, Grosse is heard responding almost immediately to Bill and with a rising inflection. Even allowing for frequency shifts due to tape-recorder play-back speed, there shouldn't be an inflection change unless the played-back version is a different recording altogether.
(at around 1h 50 mins) Hatchet marks are already visible on the door when Vic Nottingham arrives to chop through it.
(at around 1h 35 mins) When Ed goes to retrieve Janet from the closet, he burns his left forearm on a hot pipe. When he eventually gets her out and falls to the ground, the burn mark isn't there. Then, in a later scene, the burn mark appears.
(at around 32 mins) When Ed paints the photograph of the nun in the beginning, both the painting and his paint brush are completely dry.
(at around 1h 35 mins) When the sound operator, sent by the polytechnic, is recording various sounds around the house, he points his microphone at an open door leading to the garden. He picks up the sound of crickets chirping. Since this is North London in late December, the chances of hearing crickets are virtually impossible.
Cellars like the one shown were uncommon in 1970s England. If the property did have a cellar, the stairs would not be built or positioned like the way it is presented in the film. The cellar is clearly based on an American design - large and spacious - rather than the dark, damp and rather small cellars that London properties had in 1970s Britain. Those that weren't converted were filled with concrete due to flooding issues from the many underground rivers and sewers in the capital.
When Ed Warren picks up the guitar, which is implied hasn't been used for a time, he doesn't need or even try to tune it.
Fireplaces in the house are not those of a 1930s British house; they are the wrong size (more American dimensions). You'll never see a British tiled fireplace quite like that.
(at around 8 mins) In an opening montage of London images, a headline regarding the Battle of Lewisham is shown on a newspaper dated September 10, 1977. That incident actually occurred on August 13, 1977.
When Ed and Grosse walk to their car, the rest of the foreground from the 1970s depiction of England shows a couple of recent-modeled cars driving by.
(at around 8 mins) During the establishing shots of London, the song "London Calling" by The Clash is heard. The film is set in the year 1977, but this song was not released until December 1979.
While this is all true, there are no rules that state that a song can't be played on a film's soundtrack despite being an anachronism. Particularly since said song is only heard by the film's audience and not any characters of the film. Nor do any characters interact with the song, or even acknowledge its existence.
So having "London Calling" played during this scene is not really an error.
(at around 43 mins) When the Warrens' daughter is making friendship bracelets, not only is the name of the demon spelled out on the bookshelf, but the bracelet on the table to the right of Judy also says the name.
(at around 43 mins) When at home, Lorraine stands in front of a bookshelf. There are big letters marking the different parts of the small library. But instead of ABC and so on, the letters give away the demon's name. Although this reveals the name, there is no reason that Lorraine would have noticed this earlier, as it is easy to slip by unnoticed.
At 2h 5min mark Janet tells Lorraine "You said one person could change everything but I've got two" The colored contact Janet is wearing slips and partially reveals the actresses true eye color.
The layout of the house is very unusual for a UK property of this age. At the top of stairs is the bathroom, with a bedroom at back. The front of the house has a small bedroom over stairs and larger bedroom to the right of it. Also, the rooms are way oversized for the style of house shown at beginning of movie.
(at around 2h 5 mins) The song Lorraine plays on an LP at the end of the movie is track 5, side one of that album, but the needle in the shot is on track 1.
(at around 2h) During the climax of the film, when Ed is holding onto Janet outside her bedroom window, Lorraine rushes over and pulls them back into the house. "Janet" is noticeably a dummy.
(at around 1h 18 mins) When Janet is hanging from the ceiling, her hair is following what gravity normally does. This gives away that the set is actually upside down and she is really on the floor. If she were stuck to the ceiling, her hair would've been down on her face.
(at around 30 mins) When Janet is home sick from school, after she hangs up the phone with her mom who called to check on her, she uses the remote to flip back to the station she was watching, and the first station she flips to reveals an adult Jon Bon Jovi in mid-song. Since the story did not take place until 1977, Jon Bon Jovi would have been only 14 to 15 years old, a full 6 years before being discovered.
(at around 44 mins) When Lorraine is marking up the bible, Kitty Kelley's biography of Nancy Reagan can be seen on the bookshelf. That book was not written until 1991.
(at around 10 mins) The scenes outside the school are filmed in present day. The school sign is showing an 11 digit telephone number with an area code starting with 012. At the time of the film, London numbers had the prefix 01, which was introduced in 1959.
(at around 44 mins) At the Warren's home, when their daughter Judy first notices the Nun, Lorraine gets up to investigate. Though taking place in 1977 on the bar is a bottle of Gentleman Jack whiskey. This was a label that was first marketed in 1988, and further, the bottle shown is a contemporary version.
Remote controls were not standard TV accessories in the '70s, but expensive gadgets sold separately, making it unlikely that the average family would have one. Color TVs were also pricey items - half the TVs in Britain were still black-and-white sets until 1976. The idea that this struggling family would have both of these (easily pawned) devices, especially the remote, is a stretch.
(at around 1h 26 mins) While Ed performs "Can't Help Falling In Love", a crew member can be seen in the reflection of the glass cabinet.
In multiple shots looking down Green Street, there is a crossroads in the distance with a large red-colored raised speed table covering the entire junction. The real Green Street does not have this feature. The movie was actually filmed several miles away on a back street near Enfield Town.
The American investigators are seen arriving into London (and almost departing from London) at Marylebone station. In 1977, this station provided only a few regional services to Buckinghamshire and Oxfordshire, and certainly served neither of London's two transatlantic airports. At that time, trains to and from Heathrow were provided solely by the London Underground on a (then) brand new expansion of the Piccadilly Line, and trains to and from Gatwick ran (and still run today) to and from Victoria station.