In the final car chase climax, the blue car that the maniac 'sister' imposter drives to side ram Linda and boyfriend off the road, shows side front fender damage as expected from - until final shot before racing past them on the side of the road, all scrape / denting damage gone.
Rachel's homemade dress that she tries on, becomes a different dress when Julia tries it on. Rachel's original dress is a softer cotton, while the dress that Rachel is wearing, is shiny and has a single sash in the front, versus Rachel's dress that had a full bow.
About 40 minutes into the movie, Linda Blair's hair is way shorter than previously shown. Then it gets long again.
During the final car chase, when the Thunderbird goes over the cliff, it explodes into a ball of flame in midair, before hitting the ground.
Julia is driving a blue car during the chase scenes but after it goes off the cliff it is black.
The scene where Rachel says "What is it? What does she have that has you and Mike so enchanted? " Peter simply says "I guess some people are just special ". Then Peter starts walking toward the door but then turns around briefly and says "I like her.." Right after he says that he smiles right at the camera as if he is posing for a picture.
After Rachel accidentally knocks over Julia's suitcase, you can see a crew member's reflection in the mirror as Rachel kneels down to pick up the suitcase.
When arranging the flight to Arkansas, the dad mentions directions between Little Rock and Russellville. Assuming this is where the aunt and uncle were killed in the car wreck, the topography is all wrong. This area is hilly, but not mountainous and is primarily forested with occasionally large grassy fields.
When the nurse catches Rachel and Carolyn talking to Professor Jarvis, she immediately tells Carolyn to turn in her uniform and leave. A minute later after Rachel leaves, she tells Carolyn "one more stunt like that and you will be canned."
L. Blair is neither married nor engaged (e.g. to the guy who the danger stranger 'sister' imposter soon steals from her): yet throughout has a gold band on her left hand third finger.