Plot hole: According to the detective at the end Walter believes Ray kidnapped his dog, assuming he saw the note that Ray put in Walter's mail slot. But if that note was grabbed along with Walter's mail and the wig it would have burned up along with everything else when the house exploded. So how did Walter know that Ray had his dog?
Plot hole: When Rumsfeld is on his roof as a look out, he has a clear view of the neighborhood, even at night. However when the Klopeks first come home that night (and realise that someone is in their house), Rumsfeld fails to notice their car until they return again with the police. (01:20:00)
Plot hole: In the scene when the two couples go over to the Klopecs' house for their visit, the subject of Walter missing is brought up, and the Klopecs act as if they don't know where he is. But in fact, it is revealed at the end of the movie that the Klopecs were legitimately picking up Walter's mail, therefore knowing that he was in the hospital.
Plot hole: The Klopeks have only lived in the house for a month, but they somehow had a massive furnace/incinerator installed in the basement without anyone noticing the arrival of equipment, parts, delivery people, service technicians, etc. The neighbors only notice something is amiss after the new "furnace" is operational.