Anachronisms: When George's friend, Jeff, arrives at the house, he is driving a Jeep Cherokee with square headlights. The headlights on Jeep Cherokees were round until 1979/1980, therefore this type of vehicle could not have been available in 1975, when the story is set.
Continuity: The ball-shaped finial on the newel post at the bottom of the stairs (which is shown to fall off easily when the Lutz's are first being shown the house) appears, then disappears, then reappears between shots in one sequence.
Continuity: Lightning smashes through one of the attic windows (during the axe attack), but subsequent exterior shots (the escape) show both windows still intact.
Continuity: When Father Delaney's car goes out of control, the window is up/down between shots.
Revealing mistakes: When the real estate agent is showing the kitchen, she casts four shadows because of the stage lights above the set.
Revealing mistakes: During the toilets overflowing with black "goo" scene, the kids go downstairs to let Aunt Helena in. Matthew answers the door with a very red left cheek, before she gets in to start pinching. Obviously this was not the first take.
Boom mic visible: Shadow visible over Father Delaney's shoulder when the flies are on his face.
Miscellaneous: When Jackie the babysitter gets trapped in the closet you can clearly see a keyhole on the door when she tries the knob, yet later George claims it has no lock.