Continuity mistake: 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 Christmas time this is still a mistake.
Other mistake: In the scene where Kevin is making the hotel reservation on the phone, the lady does not even ask the dates of when he will be arriving and leaving the Plaza.
Factual error: No airport in NYC has the view of the New York City skyline that Kevin has when he realizes that his family is in Florida and he's in New York.
Audio problem: When Kevin plays the tape of his Uncle Frank singing in the shower, it starts with "We know a guy that can really do the cool jerk" twice. 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.
Other mistake: When Kevin first enters the hotel, he is told to get to the lobby he should go straight ahead and turn left. The next shot shows him turning right into the lobby.
Continuity mistake: When Kevin is running away from Harry and Marv he buys strings of pearl necklaces, and pulls them apart on an icy corner. When the bandits slip on them, there are more pearls there than Kevin dropped.
Factual error: When Kevin goes in the limo, the front desk manager attempts to process a charge on the credit card, which comes back as "stolen." No credit card terminal flashes "stolen" when a credit card has been compromised or cancelled. The machine would simply decline the transaction, much like if the credit limit had been exceeded, or tell the merchant to phone for authorization.
Visible crew/equipment: In the scene where Kevin lights the rope on fire, he turns to run away. Watch the window he passes, there is a crew member standing there with his arms folded watching.
Continuity mistake: When Kevin throws a rock through the front window of Duncan's Toy Chest, the alarm sounds and Harry and Marv run out of the store. In the next shot, the alarm that could be heard has mysteriously stopped. (01:17:39)
Continuity mistake: When Kevin is first watching the movie in the hotel room the room service guy already has two scoops of ice cream in the dish and is putting in a third. Then later in the scene the guy asks, "Two scoops, sir?" and only one scoop is in the dish and is just putting the second scoop in.
Factual error: The picture that Kevin takes of Marv and Harry through the toystore window is perfectly clear. There is no way you can take a picture of something through a window without the flash reflecting back and ruining the image, especially from as close a range as Kevin was.
Continuity mistake: When Mrs. McCallister receives the phone call about the police finding Kevin, as soon as she answers the phone, notice the white shirt behind her on the bed. When she tells the family they found him, the camera goes back to her and the shirt is in a totally different position.
Continuity mistake: In the scene where Harry and Marv are at the skating rink, the front of Marv's hair is constantly changing from shot to shot. It's either poofy or pulled back due to the earmuffs he's wearing.
Continuity mistake: When the robbers are in Duncan's Toy Chest at midnight, Kevin sets up his board on the bucket outside the window. He then knocks on the window, takes the robber's picture, then runs back away and pulls a rock out of his pocket to throw through it. But he wouldn't have been able to run back that far without jumping over the bucket and board, and he did not jump or avoid any obstacles.
Continuity mistake: Once Kevin and the attendant board the plane to NYC to make sure Kevin's family is on board, there is a shot of Kevin while he says, "There's my dad over there." In the background, a man is sliding his black coat off of his shoulders. Two shots later when the attendant says, "Well, go find an empty seat," in the background you will see the exact same man taking his coat off a second time.
Continuity mistake: At the pageant scene, when Kevin turns to attack Buzz, you see him make a fist; but when the camera shot changes to include the full choir and the audience, you see Kevin push Buzz with a open palm.