(at around 32 mins) When Daniela opens the glass case containing Annabelle, she leave the keys in the lock. When the smoke alarm goes off, she closes the case, but the keys are no longer in the lock.
Annabelle Comes Home takes place in 1968 (the Warrens remove her from the nurses' house after interviewing them), yet we see Rory's music box from The Conjuring (2013) present in the artifact room, despite The Conjuring taking place 3 years later.
(at around 47 mins) When playing Feeley Meeley, Mary Ellen takes out a red piece and puts it on top of the board game. In the next shot, we see it is a green piece.
(at around 19 mins) When Mrs. Faley walks up to Bob at the supermarket, she stands in front of the counter to tell him that there's been an accident. But in the next shot Mrs. Faley is standing in front of the door/entrance of the supermarket, in a completely different position to where she was standing in the previous shot.
(at around 5 mins) When Ed gets out of the car to check the engine after it dies in front of the cemetery, the headlights are off. When he is pushed by the spirit and falls down, the headlights are suddenly on.
Badfinger's "Day After Day" is on side two of the "Straight Up" album, so the label should show a cut/halved apple. The record on the turntable shows a green, whole apple, indicating that side one is playing.
(at around 1h 45 mins) A typo in the credits lists the MPAA certificate for the film as certificate #52101. This is incorrect as the correct certification for the film is certificate #52191. The actual movie with the MPAA certificate #52101 is Rob Zombie's 3 from Hell (2019).
The Crooked Man music box from The Conjuring 2 (2016) is seen in the Warren's museum. This movie take place before the events of Conjuring 2.
(at around 22 mins) Judy and Mary Ellen do not seem to be aware that the backdoor is unlocked after Daniela sneaks into the house through it and leave it unlocked for the remainder of the film, allowing Daniela and Bob to enter the house later in the film without the protagonists realizing.
The Warrens shouldn't have even needed to hire Mary Ellen as a babysitter for Judy. They could have left her in the responsibility of Georgiana, Lorraine's mother, who was established in The Conjuring (2013) to be their live-in nanny for their daughter.
(at around 19 mins) The babysitter left her eggs at the store for the cake, and her best friend took them on her behalf. Later when at the house (at around 33 mins), we see they raise chickens, and don't need store-bought eggs for a single cake.
The Warrens's daughter was born in 1950. This film is set in 1972; apparently that would make the daughter 22.
When Daniela runs in to help lock Annabelle's case at the movie's conclusion, the key is no longer on a key ring like it had been throughout the movie, but is now a single key.
The opening sequence, set in 1968, uses Freda Payne's "Band of Gold," which wasn't released until 1970.
In the first scene after the "one year later" card moves the action in 1969, Judy is seen watching Captain Kangaroo before school. But the show's intro is one that wasn't created until 1984, which is glaringly evident in its 80s-style graphics and theme song.
Bob serenades Mary Ellen with the Bread song "Everything I Own," which wasn't released until three years after the scene is set.
The story is set in 1969, yet the TV is showing the "Circle of Fear" episode "Doorway to Death," which first aired in January 1973.
Early in the film Daniela says "So not the same." Toward the end she says, "So not the same thing." Yet the film is set in 1969, and people (usually teen girls) didn't use "so" to sarcastically emphasize entire phrases until several decades later.
(at around 21 mins) Daniela stopped knocking on the front door as soon as Mary Ellen and Judy realized that there was someone at the door. After Mary Ellen opens the front door to see nobody there, Daniela appears, claiming to have been knocking on the front door for 10 minutes and that she entered the house through the back door, meaning that Daniela must have ran incredibly quickly from the front to the back door in the time she supposedly stopped knocking to when Mary Ellen checks the front door.
Set in the early 70s Judy Warren was born in 1946 whereas in this film she only looks 14 and still at school Judy would be in her 20s in the 70s.