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).
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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 Damian knocks Lee Remick over the balcony with his tricycle, five big goldfish in a bowl get knocked over, too. But the fish don't move; they don't flap around like fish do out of water. They are obviously fake fish.