Revealing mistake: When Sue Snell wakes from her nightmare, listen carefully as her mother comforts her. At one point, she calls her Amy, which is the actress's real name (they're mother and daughter in real life).
Suggested correction: Sue's mom just keeps repeating it's all right, you never hear any name mentioned.
That's because the music is supposed to drown out any dialogue.
Revealing mistake: When Carrie is hurling knives telekinetically, the first knife which hits her mother in the hand, you can see the metal protection plate that the knife actually hits instead. (01:25:55)
Revealing mistake: When Carrie is hurling knives telekinetically at her mother near the end, spot the wire attached to (and subsequently controlling) the last few knives, particularly the vegetable peeler.
Revealing mistake: After killing her, Carrie drags her mother into the closet, yet you can see her mother is walking. (01:28:35)
Revealing mistake: As the car somersaults past Carrie, a roll cannon is visible.