After Miles and Becky disable Jack, Dr. Kaufman and the police officer, Miles says, "Our only hope is to make it to the highway." This line has been dubbed in, and doesn't entirely don't match his lip movements.
Near the end of the movie, a character falls asleep for an instant and wakes up as a "pod person." This contradicts the entire premise of the rest of the movie, in which humans are physically being replaced by "pod person" duplicates.
When Miles is carrying the sleeping Becky from her house to his car, her right arm is hanging limp on the way down the stairs and her left arm is holding Miles. But in the next shot at the front door her body position is changed, with her right arm around Miles shoulder and left arm hanging limp.
As the posse runs through Bronson Cave, after the policeman
stops on the planks, says his lines, and then runs ahead, look closely for the man in the white shirt: it looks like he fell down onto the planks before the shot ended.
In the beginning, when Miles is picked up by his nurse at the train station and she opens the car's back door for his suitcase you can see the back seat upholstery is a very wide tuck & roll. A few moments later as they are riding in the car you see it is a very tight tuck.
The opening credits say :Produced in Superscope." This is not true. It was produced (i.e. filmed) spherically in the wide screen ratio of 1.85:1, then cropped to 2:1 and printed anamorphically in Superscope, despite the protests of director Don Siegel.
Upon discovering two pods in the trunk of his car after visiting the gas station, the hero pulls them from the trunk and sets them on fire with a highway flare whereupon they burst into flame. Nowhere else in the film is there any suggestion that these pods are flammable nor would they be if germinating into a human replica.
The morning after Dr. Bennell brings Becky to his house, she makes breakfast for him. She asks if he wants his eggs boiled for 2 minutes. Then they hear a noise which is the gasman in the basement, and talk to him. She then runs back exclaiming "They'll be hard boiled!". But they've only been in the water for 34 seconds. What she then serves would essentially be raw eggs.
After Jack Belicec cuts his hand, he and Dr. Bennell are talking next to the pool table. Dr. Bennell states that Becky's cousin (Wilma Lentz) thinks her "uncle and her aunt are not her uncle and her aunt". However Wilma's suspicion only concerned her uncle Ira. Her aunt was never mentioned.