When Kevin and the bird watcher go to the musical theater in which she lives, a different song was dubbed over the ones the violinists played when the camera shot the orchestra. You can tell by the bow movements and bow direction changes.
Near the end of the movie when all the kids are in the penthouse room, Buzz yells to try to quiet everyone down. He then blows on his fingers to whistle to get everyone's attention but only a faint raspberry blowing sound is made. In the next shot, everyone is paying attention to him.
When Kevin is trying to lure Harry and Marv into the house, he is seen crawling up the garbage chute. The chute was placed in a vertical position; therefore it would have been impossible for Kevin to crawl up it.
The flight attendant on Kevin's flight as well as the announcer at O'Hare Airport announce Kevin's flight as simply going to "New York" with no specifics of which airport it is. Since New York has two airports, LaGuardia and JFK (three if you count Newark), a professional airline employee would make sure to announce the correct airport especially in a multi-airport metro area like New York.
(possibly intentional) Marv tells Kevin to throw down his camera and they'll leave him alone. But Kevin's camera is a Polaroid that prints out the pictures as they're taken, so he could throw down the camera, but he'd still have the incriminating photos.
When Kevin is on the bed in the hotel room, you see the butler scooping ice cream into the banana split boat, then the butler asks Kevin, "How many scoops would you like, sir?" and the banana split boat is empty and the butler starts to fill it up.
Kevin chases who he thinks is his dad through the airport and eventually bumps into the lady at the gate and drops his plane ticket which is lost amongst the ones the lady dropped. Later in the film when he leaves the hotel and bumps into Marv and Harry (the criminals) they find Kevin's ticket on the inside of his coat; this is not possible seeing as he lost it earlier in the film.
When Uncle Frank was in the shower singing, Kevin didn't start
recording the song until Uncle Frank was halfway through. When Kevin played the song in the hotel bathroom, the whole song right from the beginning was on the cassette.
At the airport, Kevin has his father's bag as he boards the plane for NYC. Then it is by his own bag showing up at the baggage claim, in Florida, that Kevin's family realizes he's not with them. So he wouldn't have had the inflatable clown with him to use in the Plaza as a ruse in the shower. It wasn't in his father's carry-on bag, so it would've been in Kevin's bag that went to Florida.
As Kevin approaches the bathroom where Uncle Frank is taking a shower, the bathroom door is open a few inches, but immediately after that, the reverse shot from inside the bathroom shows the door shut until Kevin opens it.
When Kevin is recording Uncle Frank singing in the bathroom, he runs out and closes the door before Frank says, "Oh, you're cooking, Frankie." Later when Kevin is playing the same recording in the hotel bathroom, we still hear Frank saying this, when it should have been cut out.
When Kevin is recording the Plaza Hotel commercial with his TalkBoy and it says the toll free number, Peter walks in and asks about the battery for the camcorder, overlapping it; yet when Kevin plays it back at the hotel, Peter's voice is not included on it.
When Kevin is watching the movie in the hotel room, there is a man making him an ice cream sundae. We see the sundae first completed with three scoops of ice cream, and the scene changes to another angle of Kevin on the bed. The attendant asks him how many scoops, and there is only one in the dish.
In a long shot of the house the morning the family leaves, there is no bronze statue near the front doorsteps. But when the vans arrive, the second one hits the bronze statue near the doorsteps, which was not there in the previous shot.
As the McCallisters are running through the O'Hare terminal, an announcement about the flight to New York being in the final boarding process can be heard, with the flight's number given as 226. However, just after Kevin boards the flight to New York, the flight attendant says the flight's number is 176. Then, when we see a shot of the airport's radar shortly thereafter, the flight's number is 226 again.
In the original Home Alone, when Harry burned his palm on the red-hot doorknob, he grabbed the doorknob with his hand sideways, as if he were shaking hands with someone. In the scene when Harry and Marv meet Kevin on the street in NYC, Harry shows Kevin his hand, and the 'M' scar is positioned vertically.
At the airport, the exterior shot of the plane flown to New York is shown as a Boeing 767-200, which, unlike the 400 series 767, lacks a middle door. When Kevin enters the plane, it is clearly a Boeing 757, evidenced by its single aisle and the door between first class and coach through which he entered, which even then magically becomes an Airbus A300 cabin with the different overhead bin designs that did not exist on Boeing aircraft at the time. American has also never flown 767s or A300s between Chicago and LaGuardia.
When Marv pulls the front door knob and the knob comes loose with several feet of string attached to it. He pulls it tightly with his back against the door and we hear the first staple shot. It's impossible for Marv to do a 180-degree turn and not hear the second shot.
When Marv and Harry come out of the toy store and begin to chase Kevin, Kevin stops to get a pearl necklace to trip the sticky bandits up. He breaks the necklace apart, and a small amount of pearls falls off. When Marv and Harry slip on the pearls, tons more appear than the ones before.
When Harry and Marv run out of the toy store's broken window,
near the end of the movie, there are several shots of them where you can see into the store behind them. In one of these shots, you can see a person's left shoulder move behind a display on the left side of the screen (the person is wearing a heavy green jacket).
When Kevin arrives at LaGuardia airport he arrives at gate G6. There is no G wing at LaGuardia Airport, plus the lettering style is the same as it was at O'Hare, indicating that it was in fact filmed at O'Hare.
When Kevin gets in the limo at the Plaza Hotel, he asks the driver if he knows of any good toy stores, and he's driven to Duncan's Toy Chest. However, the Plaza Hotel is directly across the street from FAO Schwarz (on which Duncan's Toy Chest was based), so Kevin need only look outside and wouldn't have had to be driven anywhere.
When Kevin takes a picture of the criminals robbing the toy store, his pictures were clear at the end of the film. That type of camera has an instant flash, and would have made a blinding reflection off the toy shop window, revealing nothing in the finished picture other than a flash of light with a black background - not a clear picture of the crooks.
When running through the airport, a Marine in uniform walks by in the background. The Marine is in service alphas with enlisted rank on his sleeve, his cover has a quatrefoil on it, a uniform piece worn only by officers.
When Kevin goes to the front desk to check in, the desk clerk takes Peter's credit card. By regular standards, the clerk should have asked for some form of I.D., or in this case, given the fact Kevin is a minor, she should've called the police instead.
Kevin is seen buying fireworks in Chinatown when he first arrives in New York City. It is illegal to buy or sell fireworks in the state of New York, but when this movie was made, that law was not enforced in Chinatown and it was easy to find vendors there selling fireworks illegally while the police looked the other way.
In the original Home Alone, Kevin's cousin counts heads and there are 15 people between the two families. In "Home Alone 2" there are only 14 boarding passes between the two families. However, in the first movie the McCallisters travel to Paris to drop off Heather, with her parents. Therefore in the second film it is implied that Heather is in Paris during the events that take place in this film.
At the toy store when Kevin is talking to Mr. Duncan at the cash register, Duncan asks Kevin about all the money in his envelope. As Kevin is taking money out of the envelope to pay for his things he later uses to lure Marv and Harry, he places the envelope on the counter. When the camera switches back to the counter, the only things on there are the map and Swiss army knife. When Kevin requested to make a donation to the toy store, the money envelope was back in his hand. In-between, however, there is a close-up of Mr. Duncan handling the money to put in the cash register. It is impossible for Kevin to take back the envelope of money off the counter.
When the thieves are crossing the street and bump into a lady, Marv says: "mon chérie". The correct expression should be "ma chérie" ("mon" is masculine and "ma" is feminine in French). However Marv is an idiot and it's completely in character for him to make that mistake.
At Central Park where the pigeons are feeding on Marv and Harry, Kevin lights fireworks that do not scare the birds away. However, when the police come, they fire one shot in the air which scares all the birds away.
Kevin's parents questions the Plaza staff about how they would let a kid check in a by himself, but the staff for some reason never mentions the fact he had them convinced his father was staying with him.
When Kevin is in the park with the pigeon woman, he asks her if she interested in joining him in a cup of hot chocolate, which he offers to buy and treat her to. How can he do this? The Concierge back at the Plaza Hotel took not only his Dad's credit card, but also the envelope of cash. So how could Kevin buy the pigeon lady and himself a cup of hot chocolate?
When Kevin followed a man thinking it was his dad, a flight attendant lets him into the plane asking if that man was his dad. In reality, the flight attendant wouldn't let a minor on the plane with someone they don't know very well as they would have to suggest to wait for his parents to get him. Therefore, he wouldn't have been able to travel to New York.
In the scene where Marv and Harry are hastily climbing down the rope to catch Kevin, and Kevin then sets it on fire to buy him some time, prior to when they let go in the long shot, the rope is clearly not attached to the building.