Revealing mistake: As Kevin is flying off the front porch on the toboggan, you can see the small wheels mounted on the bottom of the sled to aid in its "jump". (00:25:30)
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: 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.
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. (00:14:00)
Revealing mistake: Whenever Marv is outside walking in the snow in his bare feet you can tell that the feet aren't real.
Revealing mistake: When Kevin rides the rig on the rope to get to the treehouse, you can see that it was a stuntman who is much taller than Kevin and looks nothing like him.
Revealing mistake: When Kevin screams from putting on aftershave, the shot cuts to the spider running behind the toilet. You can see white tiny pellets being thrown at the spider from behind. This must have been to make sure it kept moving during the shot.
Revealing mistake: Kevin goes food shopping, He gets milk and detergent, obviously heavy items. Afterwards we see him walking home and the bags he is carrying break open. He is carrying them like they are light as a feather. Even when the milk falls, you can tell it's an empty bottle.
Revealing mistake: When Kevin's cousin Fuller is trapped behind the kitchen chair during the spilled soda incident, the "wooden" chair back which hits him in the nose actually bends when it hits his face. (00:09:25)
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)
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.