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: 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.
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.
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.
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.