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Factual error: When Harry and Marv arrive for the first time at Kevin's "funhouse", they look up to the roof and spot Kevin taking a Polaroid of them. The flash brightly illuminates them both, but the building looks to be at least 30 feet high; a basic Polaroid camera does not have a flash strong enough to reach such a distance (plus the street is already illuminated by lamps and Christmas lights).
Continuity: When the Macalister family is in the Chicago airport and Kevin is changing his batteries, on the P.A it says that American Airline flight 226 to New York is in the final boarding process, and Kevin runs after the man who is running to the gate. Once they are on the plane, the flight attendant announces, "Welcome aboard American Airlines flight 176 non stop service to New York". These are different flight numbers.
Audio problem: When Kevin plays the tape of his Uncle Frank singing in the shower, it starts with "We know a cat that can really do the cool jerk". Yet, when he first tapes his Uncle, he doesn't start the recording until after that has been sung. The recording also sounds very clear. When he tapes him the door is closed, but the tape sounds the same throughout. Also, when his Uncle says he's gonna "slap him silly" Kevin shuts the door behind him and you can hear it shut. On the tape there is no sound of the door shutting.
Continuity: In the first film, it is mentioned several times that Kevin is 8 years old. However, in this film it is stated that he is 10. When his parents are in the Miami airport security office, they say that he was left alone last Christmas. That would make him 9. Both films take place the same days, from about three days before Christmas until Christmas morning, so even if Kevin is born around Christmastime this is still a mistake.