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.
Revealing 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.
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.
Continuity mistake: When Kevin rolls out his "Operation Ho Ho Ho" plan, similar to the "Battle Plan" from the first film, the printing on it looks to be done by a very young child, with backwards letters, even. But later, when they show the note he wrote to Mr Duncan (the one he wrapped around the brick he threw through the window) the handwriting is much neater - and even a little feminine.
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 throws a brick 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 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 yells to the family they found him, the camera goes back to her and the shirt is in a totally different position.
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 Christmastime this is still a mistake.
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: 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.
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 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 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: 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).
Revealing mistake: When Marv ends up stepping on some green gunk that Kevin set as a trap in the basement, he eventually slips and then slides into a shelf of paint. However, Marv actually started sliding after he fell to the ground. It's almost as though he was being pulled forward. How could he have slid?
Continuity mistake: When the McAllister family arrive at the Florida flight and give in their boarding passes, you can see Brooke 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: Take a look at the position of the penknife when Kevin is paying for the toys and chatting to Mr Duncan. In one shot it's in line with the camera, on its front. The next shot (10 seconds later) the penknife is on its side and lined up with the book. Mr Duncan did not touch them in this time.