Continuity mistake: When Kevin goes to the grocery, he buys Tide detergent. While walking home, the two bags break, but no Tide detergent falls out.
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)
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)
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.
Factual error: When Kevin's mom leaves Paris to return home, the plane shown departing the airport is a DC-9. No airline uses this plane for trans-Atlantic service - it doesn't have the required range.
Continuity mistake: When Kevin is leaving the church after talking to the old man that lives next door to him, he runs home. Between leaving the church and getting home he is wearing one pair of trousers, when he gets home he is wearing a pair of jeans, then when he is setting up the traps for the bad guys he is back to the first pair.
Continuity mistake: In the scene where Kevin prepares to ride his toboggan down the stairs, the front door changes position between shots from half open to completely open. (00:25:20)
Continuity mistake: When Joe Pesci places his hand on the red hot doorknob on the front door of Kevin's house, he places his hand on the doorknob sideways, but after he sticks his hand in the snow, he looks at his hand and the letter M that was burned into his hand is straight up and down, not sideways like it should be based on how he grabbed the doorknob.
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)
Continuity mistake: When Kevin is putting washing in and taking clothes out of the dryer, he places the orange bottle of liquid on top of the washer with the handle facing away. On taking the clothes out of the drier and closing the door, he turns to see the metal heater scaring him by saying "Kevin." Look over his left shoulder when the shot comes back to Kevin, the orange bottle now has it's handle facing outwards, and the top isn't on properly.
Continuity mistake: In the scene where Kevin is shooting the BB gun at action figures he has lined up along the laundry chute the baseball player figure he sets up is leaning against the left hand side of the laundry chute frame, yet when he begins to shoot at the figures the baseball player is standing up straight on its own and not leaning against the chute frame any longer.
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.
Continuity mistake: Kevin goes out to cut the top off a tree to use as a Christmas tree. The tree top he drags in is much smaller than the one he decorates.
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.
Visible crew/equipment: When Kevin first realises he's home alone and goes into Buzz's room, there is the leg of a crew member to the bottom right of the picture (wearing jeans). (00:19:20)
Revealing mistake: Whenever Marv is outside walking in the snow in his bare feet you can tell that the feet aren't real.
Continuity mistake: In the scene where Kevin is hanging up ornaments on the Christmas tree and he sees Harry peeping through the window by the reflection of the ornament, the reflection is not entirely accurate. It shows Kevin facing the camera with both the tree and window behind him. The reflection should have shown Kevin with just the window behind him and slightly to his right, based on his original position.
Continuity mistake: When Marv comes to the back door while Kevin is washing dishes, Kevin turns on the Johnny & Snakes movie. Then he goes to get the pot and puts the pot by the back door. The whole time he has his sleeves rolled up, but when he opens a drawer to get firecrackers, his sleeves are rolled down. In the very next shot they're rolled up again. The shot changes too quickly for him to roll them back up.