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: Whenever Marv is outside walking in the snow in his bare feet you can tell that the feet aren't real.
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: 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: When Marv is about to hit Harry, who has the spider on his stomach, you can tell that, for two shots, it's a fake spider. The real spider is brown. In the shots where Marv is standing about to hit it, the spider is black and motionless. (01:26:00)
Revealing mistake: When Kevin lands in the treehouse, he's at the back with his feet through the wall. Given where the rope is tied to the tree, he would have hit into the tree or the door frame of the treehouse, not at the back wall.
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.
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 and Marv are crossing the "rope" to the treehouse in the far away shots it is something thinner and resembling more like a steel cable, not a rope as Kevin had put up.
Revealing mistake: When talking to John Candy in the box van, the view outside the window screen is a projection, and the picture is not moving - at this point they're supposed to be driving.
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: 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.
Suggested correction: He does move his fingers while playing his clarinet.
Revealing mistake: When Marv is at the basement door trying to regain his stance, as he steps onto the icy ground, the "ice" bends and distorts the light just like a piece of plexiglass.
Revealing mistake: When Marv is trying to break into the basement, after he slides down the stairs and is trying to use the crowbar to hoist himself back up, the "ice" on the floor looks "too perfect" and is either a greased up glass sheet or plastic (to allow him to slide his feet around comically). You'd sort of expect the ice to be a bit uneven naturally. It's more evident in HD or digital versions.
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.
Revealing mistake: Towards the end when Harry walks up the frozen stairs, you can see it switches to a stuntman when he slips and lands on his back. (01:17:55)
