Audio problem: During the scene where Dr. Challis goes to visit his two kids, they put on their Halloween masks and his little son says "Let's watch TV." He goes to turn the knob to turn on the TV and a split second BEFORE he turns it, we can hear the intro of the commercial on the television.
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Mistakes
The story takes time the last week of October. When Challis and Ellie are on their way to Santa Mira, the trees have green leaves. Even in California, this isn't usual. See more...
Halloween III: Season of the Witch mistakes
Continuity: Right after Ellie was kidnapped, Dr. Challis runs to a pay phone and dials the Operator to get help, and begins to state, "Operator." and gets a recording. Not only does he get a recording (which would be impossible in the real world), but the recording itself states, "Your call cannot be completed as dialed. Please check the number and dial again, or, call your local operator." Problem is - he was dialing the operator.
Factual error: The "big giveaway" that happens at 9pm across the whole country would have to happen at least five different consecutive times due to time zones. The "big giveaway" in the most eastern time zone would be successful, but further west it would be earlier than 9pm.