When Peter says, "This is thing isn't dressed," he has carving utensils in the dinner entrée. In the following shot no utensils are visible when Ralph grabs the meat.
When Emily is driving the Cadillac, the point-of-view shot of the road implies that the car is swerving and traveling at a high rate of speed. The next hood mounted shot shows Emily driving very straight and moving at a crawl.
When Ralph drags in the scooter, the gate is left wide open. When Peter and Emily arrive, the gate is closed.
The architecture of the Merrye house makes no logical sense; the actual layout of the rooms and floors as they relate to each other is impossible, something director Jack Hill later admitted.
When Peter reads the textbook's description of Merrye Syndrome the following text on the page is a random patchwork of unrelated paragraphs.
When Bruno is disposing of the messenger's corpse it is obviously a dummy wrapped in the blanket as it is much thinner than the messenger's body was.
Shortly after Peter and Emily Howe arrive at the mansion, a cast member and camera are reflected in the car window as Peter rolls up the side window of the convertible.
As Virginia and Elizabeth run down the stairs from the second story of their house, the silhouette of a crew member can be seen walking around the room to the left of the staircase.
When the window comes down on the back of the messenger you can see a hand at each side of the window holding it down from the outside.
As Virginia climbs onto Peter's lap and the camera pushes in on them, the shadow of a boom mic drops down the wall.
When "dead" Mr. Schlocker is pulled from the dumb waiter, he is gripping a flashlight in his right hand. Dead people can't hold flashlights.