Revealing mistake: When Harry walks around the side of the house towards the front door, in the shot which shows Harry walking from side-on, this is obviously a stand-in, given this person is much taller than Joe Pesci (who plays Harry). It is also the same stand-in when we see Harry in the behind shots as he holds onto the railings while he tries to climb the steps. (01:14:10)
Continuity mistake: When Kevin puts down his milk and snack on the table, he is looking at the presents under the tree. When he wakes up thinking his mom might be home and calling for her, when he goes in the room where the tree is, there are no presents.
Revealing mistake: When Harry tells Marv that he's sick of him leaving the water running in the house that they robbed, you can see Harry supposedly shift his van into drive. However, Harry does not even move the gear selector from park to drive.
Continuity mistake: When Marv and Harry first attempt to break into the McCallister's house, Harry tells Marv to go to the basement, and he goes to the front door. With the front steps covered in ice, Harry slips and falls down the steps, and his crowbar clanks down at the bottom of the first step. When Harry gets up and continues trying to get up the steps and into the house, the crowbar disappears - it is not in his hands.
Audio problem: When Kate is riding in the truck with the Kenosha Kickers playing "Deck the Halls", drums are heard. When the drums are not heard, the drummer is still seen drumming.
Continuity mistake: When Kevin is shooting toys off the laundry chute, his rifle is cocked by pushing forward on the slide, then pulling back. Later, when Kevin is shooting Marvin in the pet door, Kevin pulls back on the slide, then forwards to cock the gun.
Factual error: In the scene where Kevin swipes the toothbrush from the drugstore, he slides across a skating rink on his knees while being chased by the cop. Ice is slippery, but not THAT slippery (especially when it's been skated on). Kevin would have only slid a few feet before coming to a stop.
Continuity mistake: When Marv slips while walking down to the basement, he lands on the right side of the steps. When the angle changes, he is laying on the left side.
Revealing mistake: In the scene where Kevin goes to climb the shelves, we can see that the tarantula is fake as it doesn't move, nor would its legs be spread out if it was holding onto the side of the glass tank as shown.
Revealing mistake: In the scene where John Candy and Catherine O'Hara are in the van and John Candy plays the clarinet, he never changes the positions of his fingers, but he plays a different note.
Continuity mistake: When Kevin is dancing to 'Rocking Around the Christmas Tree' in the living room, the position of the record player is straight, but in the next shot it is halfway crooked.
Continuity mistake: When Kevin is shopping for groceries the paper towels are in the cart, but in the next shot the paper towels are on the conveyor belt.
Continuity mistake: When Kevin puts the food in the casher you can see a lot of people next to him, but in the next shot there's only one person behind him.
Continuity mistake: When Mrs MacAllister is trying to get on the plane to Dallas, it is light in Paris. The movie cuts to evening scenes in Chicago. That cannot be. If it is evening in Chicago it is either night or very early morning in Paris - too early for it to be light. And when the plane takes off, it is darker than during check-in - showing that it is evening and not morning. December dusk in Paris is noon in Chicago.
Continuity mistake: When Harry slips and somersaults down the Mccallister's iced front door steps for the second time, his hat falls off at the base of the stairs while he's tumbling, yet in the subsequent shot showing Harry, he is wearing his hat again.
Visible crew/equipment: When Kevin is in the house and he locks the front door, a crew member is reflected on the mail holder.
Continuity mistake: Marv is walking up steps covered in tar in the basement. He gets some tar on his feet and we see it in the nail scene. When he falls and grabs his feet, his feet are clean from then on.
Revealing mistake: When Harry tells Marv to go back while they are climbing across the rope, in the shot facing them front-on, you can see Kevin in the background about to cut the rope. If you look closely this is not Kevin, it is his stand-in.
Continuity mistake: When Marley comes into the drug store to see Kevin, the wrapping paper is leaning on the shelves, but in the next shot the wrapping paper is gone.
Continuity mistake: When Kevin puts the spider on Marv's nose you can see a box of junk is almost halfway off the steps of the attic but in the next shot at the start of the film the box is no longer halfway off the steps.
Answer: Because he's understandably angry that Kevin's so unreasonably scared of him and assumes the worst in him without even getting a chance to know him. He never gets a chance to talk to him, because he always runs away too quickly. The first time he gives him that stare was when Kevin was watching him shovel from the window. Nobody would like to be stared at like that while doing a simple task.
MikeH