House on Haunted Hill

Deliberate mistake: When Eddie and "Jennifer" are walking around the house alone, "Jennifer" is trying to tell Eddie that she's really Sara. But By the time she is explaining that her name is actually Sara and not "Jennifer", Eddie has disappeared. She doesn't find him until later... But from the moment she finds him (and he is talking to her) he calls her Sara... but he never actually heard her say that. He was gone. Also, after they all get back together, a few of the other people start calling her Sara as well....but no one actually heard her say that her name was Sara.

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Deliberate mistake: In the scene about the Terror Incognita roller-coaster, when the rails break, they have ragged ends at the gap; after the first roller-coaster train derails, the rails move together again to close the gap and allow the second roller-coaster train to pass safely; but as you see the rails move together again, they now have neatly cut ends at the gap, not ragged ends. A deliberate error to mislead the audience, but an error nonetheless.