Revealing mistake: When Harry trips on the fan string throwing feathers all over him, the fan sound is heard running but there's no air blowing at him as the feathers are falling around him undisturbed. A few shots later, we see the fan on and blowing feathers around the room which is what it should have been doing.
Revealing mistake: The morning the McCallisters are rushing to get to the airport, the kid from across the street is bothering one of the van drivers. In the scene where the van driver says, "Gee, kid I dunno, hit the road," you can clearly see the kid mouthing the words as the man speaks.
Revealing mistake: When Marv is trying to break into the basement with the crowbar but slips on the ice, he hooks the crowbar on the sill of the door and then slips again. The crowbar then falls on his head. The mistake is that when he slips and the crowbar is hanging it is not hooked on the sill of the door, but is obviously hanging by a wire as it swings freely in front of the door.
Add timeJason Hoffman
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.