Other mistake: Kevin rides down the stairs on his sled and out the door. He originally lined up the sled at the top of the stairs, but it is clear that the stairs lie to the right of the door. His sled went straight down the stairs, therefore making it impossible to go straight out the door in one movement as shown. (00:25:25)
Other mistake: Toward the end of the film, there is a scene where Macaulay Culkin rides a rig on a rope from his house to the treehouse. Joe Pesci and Daniel Stern try to follow, but Culkin cuts the rope with a hedge cutter. Problem is: The rope is slack when Mac cuts it. With the weight of two grown men, it would have been stressed like a guitar string.
Other mistake: When Harry and Marv are making their way across the rope to the tree-house, you can see them dangling in a long shot. It has to be stuntmen because, despite the 12 inch height difference between the characters, they seem to be the same size while hanging there.
Other mistake: When Kevin is shooting the toys off the laundry chute with Buzz's BB gun, he's looking through the scope with his closed eye.
Chosen answer: They did but the power and phone lines were down due to a tree branch that had fallen on them the night before the family left. A flight attendant on their plane mentions that attempts had been made to call the McAllisters' home but "the phones are still out of order".
Jeff Swanson
But how did he call the police at the end if the phone lines were down.
That was several days later. Clearly the phone lines were repaired over the course of the film.
The film takes place over 3 or 4 days.
Ssiscool
After the storm the lineman tells them the power has been restored but the phone lines are "a mess" and that it would take several days to get them fixed. They were out initially, and then in various stages of repair afterward.
Also explains calling for a pizza.
Ssiscool