When Dr. Klopek is ascending from the basement and they focus on his shadow on the opposite wall, the shadow behaves against the laws of physics: Since he is walking AWAY from the light source behind him, his shadow should be getting smaller. A shadow only gets larger if you are walking TOWARD the light source, with the shadow on the wall behind you.
Art told Ray that he slipped a note underneath the klopeks door and Ray said he had also slipped a note underneath Walters door when in fact he actually slipped it into the mail slot not under the door.
In the breakfast scene Art re-calls his conversation with "the real estate broad that sold them the place". Later the Dr. Klopek describes killing the Nap's and taking the house from them. If this were true the house would not have been put on the market and there would not have been a real estate agent to speak with.
When the group hears the doctor's footsteps in the Klopek's living room, Carol, in the background, removes her left arm from a piece of covered furniture. In the next shot, Carol removes the same arm from the same furniture.
When Ray hits the gas line in the Klopec's basement, only a few seconds pass until he is running out of the house. There is no way that a sufficient amount of natural gas could have built up in that short time that would have resulted in such a massive explosion.
When Carol is walking up the street trying to find Ray at the end of the film, an unmarked Ford LTD police car is seen parked outside her house with its lights on. Later, she and Ray are sitting in front of it as the detective lists the charges against Ray, and the lights are off. At the end of the scene, another detective turns them on again for no reason, as the car does not go anywhere.
Carol goes away with Dave (Carol and Ray's son) about halfway through the movie so that Ray, Mark and Art can carry on with their plan to find Walter at the Klopec's house. Later, after the house explodes from the gas main that Ray broke, Carol returns without their son who left with her hours earlier. Presumably, he stayed behind with the relatives they were mentioned as going to visit.
When Art and Ray are in the basement, talking about the incubus and succubus, Art accidentally hits the lamp, causing it to swing back and forth. As the light slows down, a crew member hits the lamp again to keep it swinging.
At the beginning, when the paper boy is riding down their street, the camera gives a view straight down the street. The sign says "Mayfield Pl.", but it is positioned to show the cross street, not the street they live on.