Revealing mistake: When Harry and Marv are standing at the crossing with Kevin, they have a firm grip on him. But if you look carefully, before Kevin reaches forward to pinch the lady in front of them, Harry slightly loosens his grip on Kevin so that Kevin has enough leeway to lean forward to pinch the lady.
Continuity mistake: The woman at the gate to the New York flight arranges the tickets in a neat pile with the edges in line. The next shot, the tickets are a mess again.
Continuity mistake: Kevin is carrying his dad's bag. In the shots of him running the bag is missing, he only has his backpack, but you can see it in other shots, when the guy at the hotel is carrying his bags his dad's bag is missing.
Continuity mistake: When you see Marv first step onto the green slime in the basement, he's only a few feet away from the shelf with all the paint cans. But, after he falls, he slides for a much longer time than he would need to get to the shelf from where he fell.
Continuity mistake: Kevin sawed the ladder front to back but later when Harry Lime climbs the ladder it is shown side to side.
Continuity mistake: When Harry slips off the ladder outside, he falls backward towards the pallet on the ground (a few feet away from the ladder), but then suddenly the green slime falls from the ladder straight down onto his face. The slime would have landed on his feet or maybe legs, but by this time his face was well clear of the ladder.
Continuity mistake: Kevin takes photos of the Wet Bandits robbing Duncan's Toy Chest. Later on when the police look at a photo, it doesn't match up to when it was taken - they didn't have money in their hands.
Other mistake: At the start of the film, Kevin tapes his Uncle Frank singing in the shower, but he doesn't tape all of the singing. When Kevin is in the hotel in New York, he plays back the tape when the blow up clown is in the shower, all of the singing is on the tape, including something his uncle said when Kevin ran away from him.
Other mistake: While Kevin asks the ticket agent in New York what city he's in, there are signs at the gate indicating departing New York LaGuardia and destination Los Angeles. No gate at any airport would have a "departing" city sign - only destination city. Travelers know what city and airport they are physically at and leaving from.
Revealing mistake: This film takes place around Christmas, however, when the airport shuttle vans leave the McAllister house to head for the airport, there are short shadows under the vehicles, indicating it was filmed during the summer.
Other mistake: The old man that receives Kevin's bag in Florida can be seen walking through the airport when the family runs past him. The man could not have boarded the flight to Miami, as the family ran past him and were the last to board. The man was wearing shorts when he received the bag (but he could have changed on the flight in theory - had he been able to board). An old woman with him was in the same situation. (00:12:52 - 00:17:36)
Suggested correction: The man did not receive Kevin's bag, and there is no confirmation that it is the same man.
Revealing mistake: Just after Mrs. McCallister screamed "KEVIIIN!" as she realised his son is lost again and faints, there's a little girl in the background who looks at her falling backwards and then at the camera for one second.
Continuity mistake: When Harry and Marv are climbing down the rope, Marv goes first. There is a low shot looking up at them and you can see Harry's head look over the roof. The next shot is looking right at Marv as he starts to climb down. Harry has vanished.
Plot hole: There is absolutely no way that a hotel staff employee would ever let a child check in alone. The staff would be required to personally contact the owner of the credit card Kevin hands them personally to make a reservation, not to mention Kevin never had his dad's government I.D on hand, so there was no proof of verification whatsoever, which is also required. Not possible today or even back then, especially considering the lawsuits, terminations, and bad press that it could lead too.
Continuity mistake: When Kevin checked into the Plaza, the credit card was manually processed by the receptionist. However, the Concierge later runs the same card electronically and the screen flashes "stolen." The hotel wouldn't have both systems in place for credit cards. They would have had to call the credit card company to verify the status if they used a manual processing system.
