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.
Continuity 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.
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.
Revealing mistake: When Tim Curry runs out of Kevin's hotel room after being scared by the inflatable clown doll he hits his knee on a table. When he hits the table and then hops away you can clearly see that he is wearing a knee-pad.
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.
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.
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 McCallister family arrive at the Florida flight and give in their boarding passes, Sondra standing in front of the gap between the man and lady taking the tickets, and then leaving. In the next shot she is standing in the same place again and leaving a second time.
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.
Other mistake: When the Wet Bandits are stealing the charity money from Duncan's Toy Chest, Kevin takes their picture twice, but only one of these pictures comes out of the camera.
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.
Continuity mistake: When Kevin bumps into the lady at the airport you see the guy who Kevin thinks is his dad run through the door and out of sight. In the next shot, you see him walking through the door.
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.
Revealing mistake: In the scene at the end when Marv and Harry are talking to Kevin on the roof, if you watch it when Kevin throws the brick down at them, the one brick hits Marv. They cut film somehow so you wouldn't notice easily, but when it hits Marv you can see the corner buckle in, as it's a sponge made to look like a brick.
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.