Bobby falls on the floor in the basement, dropping his recorder. In the background, amongst the many whispers of the ghosts, Jean warns Bobby to get up and leave the basement. Right after that, a voice whispers in the background, and the caption reads "Leave me alone." However, with the volume turned up, the voice (belonging to the Jackal) clearly cackles a sarcastic 'Listen to your mother!'
Dennis states that anyone who touches him, has their whole life flash before his eyes. Yet Maggie clearly touches him on the back several times while in the basement, and nothing happens to him, not even a twitch.
When Kathy is attacked by the Jackal, he claws at her right breast and it falls out of the bra (but her nipple covered by a scrap of her shirt). In the next shot, her breast is back in the bra perfectly.
In the opening when Cyrus tell Denis where's The Juggernuat is, and after when Denis point where he. And it flashes but if you have a quick eye or slowly rewind it you can clearly see the man holding a cutter board.
Dennis said that the walls of the house were soundproof, but when he is trapped in the room with one of the ghosts, Maggie is yelling at him to move and duck. He can clearly hear her, and she can hear him, even though he isn't supposed to be able to.
Kalina tells Arthur that she slipped through an opening in the house, and that it was probably closed now. This is confirmed by the shot of the house being completely sealed with the metal plating. However, there are several shots in the basement where you can see a glass window with a tree branch/bush outside.
The goof items below may give away important plot points.
Dennis wears glasses as part of his 'repair guy disguise' and loses them in his first trip to the basement, never bothering to look for them. But he also starts the junkyard scene with them and also gets them back when he becomes a ghost, proving that he actually wears glasses. Why do never see him squinting at anything?