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.
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). (01:23:05)
Continuity mistake: When Marv has just climbed out of the basement through a hole in the floor, we see him dangling facing forwards, but the shot of his legs below gives the impression he's facing sideways. He has no time to switch positions between both shots.
Continuity mistake: When the McCallisters are at the police station, the cop puts downs the phone and tells them there is "No sign of him." Behind them, the coffee pot is half full and a lady in a multi-coloured top talks with another cop. In the next shot of the McCallisters, the coffee pot is almost empty, a coffee tin with a red lid appears (that was taken away in an earlier shot) and a completely different lady talks to the cop behind them.
Revealing mistake: In the whole scene where Harry's head is caught on fire to the part where it shows the explosion outside the building, this is quite obvious that the whole scene was done during the day.
Continuity mistake: At the end, Harry walks to the building's door, thinking he can get in. The bag with tools opens and tools fall all over him. After a few scenes, he gets up and you can see that the tools are in one place and not scattered anymore. Plus the bag and string have disappeared.
Continuity mistake: When Kevin is checking into the hotel you can see a pair of legs in the top right hand side of the screen. At one point they walk away and then are back in the very next shot to continue standing while Kevin talks to the hotel lady.
Continuity mistake: When Harry is going to the back of the relatives' brownstone the row of houses is on the right as he walks away from the one Kevin is in. When he gets to the back the houses should all still be on his right, but the one he breaks into is on the left.
Continuity mistake: Before Harry jumps for the ladder, there are several close-ups and there is nothing on the bottom rung. When Harry jumps, the bottom rung is suddenly covered in green goo causing him to slip.
Factual error: The AA flight number to Miami is a 4 digit number beginning with a 5. These are regional flight numbers but the family flew on a mainline 767.
Continuity mistake: Kevin asks the limousine driver, "Do you know any good toy stores?" His glass of Coke is almost empty. When the driver says, "Yes sir.", it is three-quarters full.
Continuity mistake: When Harry goes up the steps leading to the back door of Kevin's uncle's house, the wooden planks he kicks away are in a different position to when he first enters the alley.
Audio problem: When Kevin is singing on stage and Buzz is playing air drums by Kevin's ears, Kevin's singing is out of sync with the audio.
Continuity mistake: When the vans from the airport arrive, notice the second van to arrive smashes the statue in front of the McCallister's door all over the pavement. When the whole family is running out of the house into the vans, the statue is standing perfectly again like it hadn't been touched at all, but pieces of the feet were broken off, so it couldn't have just been stood up again.
Continuity mistake: When Kevin is running from Harry and Marv he breaks a fake pearl necklace to spread the beads all over. You can see after he drops them all that they have spread far apart. In the next shot where Harry and Marv's feet are shown there are a lot more beads all together which is why they slipped on them.
Continuity mistake: When the bag of plaster is falling downwards on Marv, in the overhead shot, the pile of rope laying on the floor in front of Marv, is not visible as it was two shots previously.
Continuity mistake: The hotel staff and security come up to Kevin's room - Kevin starts playing the tape and fast forwarding and muting when the woman in the scene talks. At the end of the scene when the man in the movie says "maybe I'm off my hinges but I forgive you" the woman lets out a sigh of relief. Kevin leaves as he plays this scene, and the sigh in the movie couldn't be heard.
Continuity mistake: Kevin's mom goes to the 3rd floor to lecture Kevin. When Kevin stands they move shots between mom and Kevin who is standing still. There are 3 separate shots of Kevin in this sequence. In the 1st & 3rd shot his left collar button is covered and his hair bang is above left eyebrow. In the second shot the left collar button is visible & the hair bang is lower touching the left eyebrow.
Revealing mistake: In the scene where Kevin runs into his relative's abandoned house in the middle of the night to set the traps, he runs down a ramp. You can tell that shot was filmed in broad daylight.
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.
Chosen answer: Remember that Kevin didn't give his name, nor did the police ever see him. When he called, it was from a different address and he gave his name as "Murphy." There is no reason to think that Harry and Marv would tell the police about Kevin, since they would have to admit being outsmarted by a child and also admit to kidnapping, assault, battery, and a wide array of charges for what they tried to do to him. Even if he did testify, there is no reason to think his parents wouldn't know.
Greg Dwyer
And by the way Kevin knew nobody would believe him.
Not entirely correct - He rang the Police from the phone in his parents bedroom just before the spider scene.