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.
Revealing mistake: In the scene when the bandits are in the uncle's house at the bottom of the stairs, the tool chest falls down. When it hits the door and they slide back if you look closely you can see that they are on rollerblades.
Revealing mistake: When Kevin arrives at the airport in New York (filmed at Chicago O'Hare) and goes up to the agent to ask her what city is outside the window, one of the destinations on the board at the gate is New York - LaGuardia...giving away that the scene was not shot at a NYC area airport.
Revealing mistake: Makeup is visible before the first brick hits Marv Merchants.
Revealing mistake: When Kevin breaks the pearl necklace while being chased by Harry and Marv, the hands doing the breaking are not Kevin's, they are much too big to be his.
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.
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.
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.
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.
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.
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.