When they are flying the plane it appears to be landing halfway through the scene, then suddenly its high up again in the next shot.
The murderous couple are using a rope tied to an airplane to drag a potential victim. He is running to keep up before they take off. In a shot from behind the plane, he falls down. The next scene, the plane is in the air and he is hanging off the wing.
As the couple are running from police, they pass a hippie strumming a guitar. This character was earlier blamed for a number of killings and would be in custody.
Obvious difference between real and doll Patricia when she is killed by a bulldozer.
Leslie clearly moving her eyes after being burned and overdosed. This happens when inspector finds her as well.
When Foster is shown hanging on to a plane from farther shots, it's clearly a doll.
When shepherd throws Christopher into the pit, Christopher ends up buried underneath the stones, which wouldn't happen if he was just thrown onto them.
At numerous points in the movie sound doesn't match the visuals and there is no mouth movement of the characters or it's unsynchronized. This happens when Celia asks Richard to postpone a murder, when they are running at the beach after riding a motorcycle, when they are riding a motorcycle in town after discovering Foster, when he calls for Patricia in her house while she is taking a bath.
When they are in the phone box you can see a cameraman in the reflection of the glass door.