Just before Ed Warren and Lorraine Warren visit the Perron family on November 1, 1971, the family is shown watching The Brady Bunch. According to the clock on the mantle, the time is 1:55 pm, but that show did not start syndicated afternoon reruns until 1975. Furthermore, the episode shown is The Brady Bunch: A Room at the Top, which first appeared during prime time in March 1973.
At the end of the movie, the Warrens' mention that they have to go to Long Island for a case, referring to Amityville. However, the movie is set in 1971 and the Amityville case didn't happen until 1976/1977.
When Judy is walking down the stairs, it is a very low shot and a modern disc-shaped smoke detector can be seen above her on the roof. In the early 1970s, smoke detectors would have been beehive-shaped.
A few minutes into the movie, the camera pans across the front porch. Visible along the base of the outside wall is a RJ-11 phone jack with a spring loaded cover. RJ-11 phone jacks were not used in 1971. RJ-11 jacks were introduced by phone companies in 1976.
In the beginning of the movie, when the Perron parents are in their bedroom looking out at the dog, it is night time and they have the lights on. When the view changes to the outside looking up to their room, it is clear that only one window is illuminated from the light. The next scene, Carolyn wakes up and two windows are visible in the room.
When Ed and Lorraine are driving to the Perron's house for the first time (after Carolyn begs them to come), you can see that the leaves are green. It looks like late spring or summer. But when Ed makes the tape of the occurrences in the house, he states that it is November 1st. The leaves would certainly have been orange/yellow by then.
Early in the movie, the piano is in the cellar at the bottom of the stairs. However, when Carolyn goes down the stairs later in the movie, the piano isn't there. Later, when the investigators go down to the cellar, the piano is again at the bottom of the steps.
Throughout the movie, when the music box is being played, the clown in the music box is shown to move up and down with the music. However in the final scene of the film, when the music box plays the clown doesn't move.
When Ed and Lorraine enter the Perron's house, Carolyn tells them that the family now sleeps downstairs. When they walk into the living room, mattresses are seen on the floor. When Drew is setting up cameras in the bedrooms, there are mattresses on the beds. During the night shots, the beds are stripped of bedding and mattresses.
When Rory's ghost leads Cindy up the stairs and into the bedroom, Drew is listening through the audio equipment and says there is someone else in the bedroom with Cindy. When Roger, Officer Brad, and Ed look for Cindy in the bedroom, Ed tells Officer Brad to get the UV light from Drew. When Officer Brad asks Drew for the UV light, Drew acts as if he hadn't heard it despite still wearing the audio headset.
When Lorraine is trying to see Rory through April's music box, the little dial used to make it play music changes positions throughout the camera angles. The dial doesn't move when playing, and when Lorraine is finished spinning the dial, it is in a vertical position. When the camera changes to look over Lorraine's shoulder, the dial is in a horizontal position. This changes back to the vertical position when the camera returns to Lorraine.
Around the 1:35::45 mark (on Netflix at least), Drew is running around the house looking for April. Just after he comes down the stairs and slowly enters the living room, on the right side of the screen you can see a crew member in a plaid flannel shirt just as he rounds the corner.
The television remote control shows that it is Ceefax capable, something only prototypes had, which became available in 1977. This wasn't on a regular household remote control for a TV. Also, TVs with remote controls would have been very expensive, and the Perron family are impoverished.
On the landing to the basement stairs, you catch a glimpse of a vacuum cleaner that is a Kirby model 1CB Omega. That model of Kirby did not appear until 1972, a year after the movie was said to take place.
During the scene where Ed first paints the portrait of the Nun and he and Lorraine go into the kitchen, the name Valek is written in the kitchen in several spots. Notably the windowsill, where it is spelled out in individual letters on colorful shapes and then on the left of the screen as the camera pans toward Lorraine as she sits at the kitchen table.