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.
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.
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.
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).
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.
The goof item below may give away important plot points.
At the very end when Matt and Jonah become one, Matt grabs the axe and Wendy sees Matt coming across the lawn. Wendy closes the door and tells Mary to go find a hiding spot. Matt breaks down the door, grabs Wendy and Billy and pushes them out the door. Once outside, as Matt says "Whatever you do, don't let them put out the fire", Mary can be seen standing with Wendy and Billy.