When Carol is showing Allan the car before the birthday party, he has a light colored polo shirt and trousers. When they go back into the house, he is suddenly wearing a dark colored shirt and trousers. When the leave the party and find the car has flat tires, he is again dressed in the light colored outfit. And when he returns to the party, he's back to dark colored.
Throughout the movie, Carol is shown with dark hair. But at 1:02 when the family is outside at the car with flat tires, her hair is obviously blonde. At 1:04 when they return to the party, it's dark again.
Its suppose to be a desolate bleak area, but there are palms, greenery, and a river to swim in.
When Sandy is leaving the kitchen to go swimming, as in some other similar shots, the door which supposedly leads outside obviously opens to a studio interior. This is also the reason why the kitchen windows are always curtained, and the outside view is never visible through them.
The mute known only as Him, is wearing a ring on his ring finger, which would indicate that he was married. Near the end of the movie, the ring is gone.
On some occasions, especially when Duke attacks Carol, we hear it bark but its mouth stays closed.
When Allen is talking to old farmer Ben, their mouths are not moving. The voices must have been dubbed in for this scene.
When "Him" (Carl) is carrying Sandy down into the pit where the spaceship is, the shadow of the boom mic can be seen on the ground to the left of the screen.