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.
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.
Continuity mistake: When the robber with the beard gets hit in the face with an iron, the iron hits him on the face, and leaves a mark slanted to the right. It also leaves a mark on his nose. When the two robbers are talking after they have been tricked and before Kevin puts the spider on one of the robbers, the iron mark is vertical and there is no mark on the nose anymore.
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 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.
Continuity mistake: Harry and Marv both get hit with the paint containers on the stairs. Look at the front doorway when Marv gets hit. The cord (from the hot implement attached on the doorknob) is in an 'S' shape and lots of micro machines litter the doorway. Harry then gets hit. When he gets up to go up the stairs after Kevin, the cord has moved and the micro machines are well clear of the doorway.