Continuity mistake: This makes the second time that this mistake - discussed earlier - occurs. After Klopeck's house blows up, Carol Peterson drives back into the neighborhood to notice all the commotion. The first camera shot (shown from inside the vehicle) shows the front passenger's head rest to the seat is missing. Next camera shot (shown from the outside of the vehicle) shows the head rest as being back on the passenger's seat.
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: 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.