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Anachronisms
At the beginning of the film, the father mentions that he would look the house up on the Internet. The Internet was not widely used for consumer or commercial use during this time. The Internet did not become used for such commercial use until the mid to late 1990s.
When Wendy is in the shower, the curtains wrap around her. When the curtains are removed, her once wet hair is nearly dry.
When the Reverend took out an eyelid from the box to look at it, it vertical, when the shot is from over his shoulder the eyelid it is horizontal. Yet back to a side shot, it is once again vertical.
Corpses' eyes dessicate noticeably and morticians as a matter of routine fill out eye sockets with cotton wadding for open-casket funerals. As a result, corpses that have had their eyelids cut off would not have the natural looking eyes portrayed here.
After receiving radiation treatments, Matt should always have a reddened neck and chest. After treatment his neck shows no redness. Later we see redness when he is shirtless.
When Wendy sends the kids to play hide-and-seek and Mary goes up to the attic, she leaves the door at the foot of the stairs open. When Mary falls through the floor and screams, Wendy runs to the attic and yanks open the door, which was closed before. However, due to the strong spectral forces residing in the house, the ghosts could have possibly closed the door. We know this because later on in the movie it is shown being done (doors opening and closing).
At the beginning of the film, the father mentions that he would look the house up on the Internet. The Internet was not widely used for consumer or commercial use during this time. The Internet did not become used for such commercial use until the mid to late 1990s.
When Matt is in the hospital waiting room, there is a TV set up with a VCR. The TV an 80s model and does not look out of place given the film's time period but the VCR is a model from at least the late 1990s.
The family's station wagon shown since the beginning in the movie is newer than 1987. It's a Pontiac 6000 and the front end shows a light and grille style that was used from 1989 to 1991.
The film takes place in 1987, but the ambulance shown immediately at the beginning is much newer than that.
When the family is huddled on the bed, you can see reflections in the brass rails. (There is no fireplace lit [fire reflection] and no ghosts are present at that time.)
After Matt escapes the hospital and returns home, Wendy tells Mary to play hide and seek. After Mary runs away to hide Matt finds Wendy and Billy then forces them out of the house. Matt then sets the house on fire. When the parents, reverend, and police show up neither Wendy, nor Billy, mention that Mary is still in the burning house. Not even after the firemen show up to put out the fire does anybody mention Mary. The mother goes in the house to save Matt and is then rescued by the firemen. After Matt and his mother are rescued from the fire nobody mentions Mary. Mary is not seen, or mentioned, after she is told to play hide and seek.
In 1987, the husband sells his hot rod truck and buys a 1982-1987 Buick Regal that already has decades worth of rust/wear on it. His buys this "old beater" to save money but in actuality, a Regal of that age might even be worth more then his collector truck because it's practically a new luxury car.