Continuity mistake: At the end of the movie when the Lutzs' get in their van and drive off; they stop near a dead end sign when George gets out and goes back to get Harry. When he and Harry get back to the van and drive off, the van is farther from the dead end sign than it were before.
Other mistake: When George is in the basement, his son starts walking down the stairs. Suddenly a light bulb shatters right next to the boy's head, he screams, and falls violently down the stairs. But when it cuts to George coming over, he's just walking at a normal pace as though nothing happened. (This is just after they move in, before the evil spirits take hold of George, so there's no reason for him not to be alarmed.).
Continuity mistake: In the beginning, when it shows the outside of the house, we hear Ronald DeFeo killing his family with a gun. But during the house tour, when it shows the murders from the inside point of view, the number of gunshots fired is different than when we heard it from the outside of the house.