Continuity mistake: When Art is eating breakfast at the Peterson's house, he takes items from the fridge. Watch when he comes back over and talks to Ray. The position of the pineapple under his arm changes from shot to shot. (00:10:40 - 00:11:25)
Revealing mistake: After Art and Ray successfully blow up the Klopek's house, Ray emerges from the ruins and the porch collapses behind him. It can be seen that the porch supports are dragged backwards towards the house to allow the porch to collapse behind Ray.
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. (01:25:10)
Continuity mistake: At the Klopecks, the bonfire of 'romantic' candles changes with each shot, from big candles to severely melted ones back to full ones again, a few times throughout the scene.
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)
Revealing mistake: When Carol suggests they introduce themselves to the Klopeks and Ray respends you can see Rick Ducummon in the background failing to stifle a laugh.
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.
Continuity mistake: When Art jumps the fence to Ray's property to escape Landru, the Great Dane, his feet break the alarm trip wires as he falls. Later, when Art, Ray, and Mark are going over the fence after Art cut the power, the alarm trip wires are intact again.
Continuity mistake: When the garbage men are first seen discussing attending the healing seminar or taking on Roselli plumbing, the house in the background is different from the house on the street across from the Klopek's house which is then seen after Art, Ray and Rumsfield go out to question them about the trash.
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.
Other mistake: When Walter had to go to the hospital, he asked the Klopeks to pick up his mail. However, a mail slot is built into Walter's front door, so the mail carrier would drop the mail through there, directly into the house. There was no need for the Klopeks to gather the mail and keep it at their home. There was no outside mailbox, and it was established that the Klopeks went inside Walter's house. If anything, the mail should have been picked up off the floor and left on a table.