The night Harry and Marv break in to the house, Kevin sits down and blesses his macaroni and cheese dinner just before the stroke of nine. This draws obvious attention to his plate. Later, just before Harry is covered with feathers, you can see a completely different plate with three sections of food on it.
Kevin has a pizza delivered to the house even though the phones are supposed to down. There is never any indication of if or when the phones were restored.
When Kevin goes across the street to his neighbors house entering though the flooded basement his clothes are soaked as he rushes up the stairs. In the next scene where he goes though the door to run into Harry and Marv his clothes are dry.
When Kevin leaves the church after talking with Old Man Marley, he's wearing green pants. As he arrives home, he's suddenly wearing blue jeans. Then as he's setting up the traps, he's wearing green pants again.
When Harry grabs the heated doorknob with the "M" engraved on it, he grabs it with his knuckles at 9:00, facing left. After he puts his hand in the snow to cool it, he looks at it and sees the "M" upright on his hand.
When Kate calls the police from France, the woman answers the phone "Village police," but when the officer goes to the McCallister home, he clearly has a City of Chicago flag on the right shoulder of his jacket.
When Harry & Marv are hit by the paint cans the next shot shows Kevin standing several feet to the right of the stairs with the ropes attached to the bannister. There is no way they could swing straight down the stairs from where they are attached.
Little Nero's Pizza's guarantee is that the pizza is delivered within 20 minutes (presumably after making the order), or it's free. Since the average medium pizza takes approximately 15 minutes to prepare and cook, this is a ridiculously unreasonable guarantee. Most pizza joints guarantee delivery within 40 minutes.
Commercial flights from North America to Europe depart in the evening, not in the morning or afternoon.
The plane shown landing at the Scranton Airport is a McDonnell Douglas DC-10. The Scranton/Wilkes-Barre Airport has never had regular service from a DC-10 by any airline. The largest planes to land at Scranton are Boeing 737s and Airbus A320s. DC-10s are far too large to be accommodated by the airport for non-emergency situations.
When Kevin is having a flashback of his family yelling at him, we see Linnie turn around and go down the stairs. When this actually happens, we don't see her turn around at all. However, the other flashback visions were also not shown exactly as they had happened earlier in the film. They were all a little exaggerated as usually happens in a child's mind but could also be a result of the frailty of human memory in general.
The house loses power in the night from the falling tree branch. This causes them to oversleep. The power gets restored before they leave, obviously, because we see the parents' alarm clock showing 12:00. The timers for the lights would be affected, and the lights would not turn on at the same time that the burglars were expecting. However, Kate asks Peter on the plane if they set the timers on the lights. Peter says "I did." This indicates that they did in fact re-set the timers.
In the "Kevin's plan" montage, Kevin hoses the outside steps down to the basement. In the next shot he is painting tar on to the inside basement steps, working from the top down. It may seem he has effectively painted himself in to a corner and now has no way back out of the basement, but it's possible that either it took place before watering the stairs outside to the basement or the water probably hasn't frozen yet making it safe for Kevin to get out of the basement.
As Harry is driving down the driveway after a burglary he is wearing his black knit cap. Just after he hits the brakes to avoid hitting Kevin he pokes his head out to tell Kevin to be more careful and his cap is missing. However, he can clearly be seen removing his hat as he is winding the window down to talk to Kevin, which is why he is not wearing it in subsequent shots.
There is no explanation about why Old Man Marley had a bloody rag on his hand while in the store or BandAids on his hand when he and Kevin shake hands in the church. However, the novelization by Todd Strasser explains that when he was in the store, he cut his hand on the blade of his shovel by accident. Even without the novelization's explanation, it is still possible that he had blisters on his hand caused by shoveling snow which burst, necessitating the bandage.
When the McCallisters are leaving in the morning and the neighbor boy is talking to the van driver, he is mouthing the driver's lines (the first time the driver speaks).
After Marv is hit with an iron, a wrinkle can be seen in the mark on his face revealing that the iron print is actually a plastic sticker. It can also be seen later when Kevin puts the tarantula on Marv's nose.
The iron Marv pulls down has round holes, but the imprint it leaves on his face has rectangular ridges rather than round ones. Since the ridges on his face are caused by the holes in the iron, the shapes should match.
Obvious stunt double in the long shots of Kevin swinging across to the tree house.
When Kevin cuts the rope in the treehouse, the rope clearly has far too much slack for even a single person, let alone Harry and Marv, to be hanging from it.
After checking too see if the cars are in the garage, when Kevin comes back into the house, the tree in the background is full of green leaves.
Kevin takes a risky act using his sleigh to propel down the house stairs and out of the open front door. He never considered he injuries that might result from his actions. These include falling off the sleigh and damaging himself on the way down. This would result in in hospitalization or even death if he falls off and hits his
head on an object and even pop his neck with this act. The sleigh might not move in the correct direction, causing it to hit the stairs or the door frame. Luckily, there was no harm.
When Aunt Leslie says "Fuller! Go easy on the Pepsi", it is quite clearly a close-mic voiceover recorded off set (most likely in post-production), as the rest of the dialogue has a completely different natural reverb to this line.
Near the end of the film, as Kevin looks out of the window to see Mr. Marley reunited with his son, a hand holding a camera is clearly visible.
When Kevin walks into Buzz's room looking for his family after being left alone, on the bottom right of the frame you can see a crew member's leg.
While Kevin is holding shears to cut the rope in the tree house, a few inches ahead of where he is to cut you can see something that looks like fishing line that is holding up the rope. This line is visible twice, as he appears with the cutters and again just as he is cutting the rope.
When the police officer is chasing Kevin, the shadow of the camera is briefly visible on Kevin's back.
When the plane takes off from Chicago, it's morning. It takes about seven or eight hours to fly from Chicago to Paris, and Paris time is seven hours ahead of Chicago time. That would put them in Paris around one or two in the morning. Yet when the plane arrives in Paris, it's already broad daylight.
When Kate phones the Chicago Police from France, she calls them on a payphone that is a BT model that wouldn't be found in France.
When the whole family arrives in Paris (which is really the underground walkway under the O'Hare Airport Hilton that connects the terminals at O'Hare, and looks like it may include parts of the old Terminal 4), Kate uses a public phone to call home. But this device
can't be a French one because all phones available in France in the 90s required telephone cards.
As the family bids farewell to Mom at the Paris airport, tail colours of an Eastern Airlines jet are visible outside the gate window. Eastern was a domestic U.S. airline and would not have serviced Paris.
Kevin wouldn't have enough time to completely refurbish the house after his battle with Marv and Harry.
It seems strange that Kevin would include the Christmas ornaments as part of his battle plan, as they would only hurt someone who no longer had shoes on and Kevin had no way of knowing Marv would enter the basement first, leave his shoes on the stairs, and only then enter through the window.
The phone lines are down in Kevin's house and online ordering was not available in 1990. The movie never explains how Kevin ordered his pizza.
Kevin's mother could have just rented a car herself, instead of getting on the bus with John Candy.
When the family arrives at the gate for the plane, the door is already shut. Once an airlines shuts the gate door, they don't reopen it. Anyone arriving after the door is shut cannot board the plane.
Peter calls the French police from Paris and is using a translation book, getting frustrated that the person on the other end of the line can't understand him. However, Peter's brother Rob, who lives in France and likely knows at least a little French, is in the next room and could help translate.
Marv gets more injuries than Harry. For some reason, after getting a shot to his forehead, he smartly smashes a side window and climbs in only to step on glass ornaments. Some diagnosis says that's the least injury he should worry about. Others say the glass would become embedded in the foot and would need to be removed, piece by piece, which would prove very difficult. He would not be able to walk after this point, the embedded pieces would go deeper and deeper, cutting his foot with each step. Apparently, he can.
Kevin had many opportunities to get help from others like old man Marley and other people in a church, he even had access to a phone which seems to be working by the time the burglars start attempting to enter the house (by that point, Kevin had ordered a pizza for delivery). Yet all Kevin does to get help is call 911 to report a robbery at a neighbor's house.
When everybody is home near the end, Kevin tells his mom and Dad he bought milk, eggs, and fabric softener when he went to the store. However, in the earlier scene at the store, there is neither eggs nor fabric softener among the items he bought.
When Kevin arrives home after speaking to Old Man Marley at the church, he runs right in the house without unlocking the front door - he left his house unlocked even though he knew that there were burglars in the area.