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.
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.
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, but in the scene immediately before, Kevin's father was trying to reach Kevin.
When Heather is told to count the kids before they load the vans, she counts herself as number three. Despite placing her hand on the sixth boy's head, she fails to count him as number six, as she is distracted by Buzz's attempt to make her miscount. She counts Buzz as number six instead, missing the boy out completely. She counts the neighbor boy in place of Kevin, but then counts herself again as number eleven, which appears to make up for her original mistake. Both times she touches herself on the head to prove this.
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.
When Kevin goes across the street to his neighbors house, entering though the flooded basement, his clothes are soaked 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.
Immediately after landing in Paris, Peter McCallister leaves a message on the Murphys' answering machine while Marv and Harry are burglarizing it. The phones are still supposed to be down, and since the Murphys live across the street from the McCallisters, their phone service should be down as well.
When Kevin's father calls the Murphy home while Harry and Marv are robbing it, Marv picks the entire phone off the table. At this point a receiver is visible on the phone. In the next shot of Marv, it is gone. If it was in fact supposed to fall off, the answering machine would have been cut off anyway.
While Kevin is in the kitchen with the whole family. Buzz is clearly stuffing the last piece of Kevin's pizza in his mouth. In the next shot he is shown with half a piece of pizza in his hand and his mouth is not stuffed.
Kate tells the pizza boy that her brother-in-law drove in from Ohio earlier that day with his family, yet later in the film only Kate and Peter's cars are seen in the garage. There is no other vehicle parked anywhere near the house.
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.
After Harry touches the heated doorknob and Kevin flees from the door at 80:48-80:51 on the DVD, the charcoal lighter cord runs out of the foyer into the next room from which the camera is shooting. Later, when Harry and Marv are hit with paint cans at 84:18, the door and the red-hot charcoal lighter can be seen in the background, but its cord does not leave the foyer.
When Kevin walks up to the church at night, there is a door clearly in the very center of the building. When Kevin is shown inside the church, there is a baptistery in the center where the door was on the outside.
When Kevin is at the top of the stairs getting his sled ready one shot shows him throw his scarf over his right shoulder and it slips down his arm. In the next shot, from behind, the scarf is clearly thrown behind his shoulder.
When Harry slips down the stairs while trying to go in through the front door, he drops his crowbar and doesn't pick it up, but in the next long shot the crowbar is no longer on the ground right next to the steps.
When Marv goes to look and see if anyone is home, Kevin is washing up and has his sleeves rolled up. He plays "Angels with filthy souls" and gets fire crackers out. He reaches into a drawer to get a lighter and we only see his arms where his sleeves are down. When you see a full view of him, his sleeves are rolled up again
When Kevin is watching TV in the kitchen, there is a gingerbread house next to the TV in shots directly facing the TV. But when the shots are from the side (on Kevin watching TV), the gingerbread house is not there.
Throughout most of the film, the pictures hung on the wall going up the main staircase are consistently fifteen in number, with John Heard and Catherine O'Hara (Peter & Kate) being the lower-most bottom-row photos, and Macaulay Culkin (Kevin) being the one at the very top end, by itself. However, in a couple of scenes there are only 14 pictures - no Kevin up top; completely different (non-closeup) photos where Pete and Kate should be; and several other pictures are different too. (The two scenes are: When Kevin is crazily running around celebrating his newfound freedom (22:23), and later when he's running upstairs screaming after having seen Old Man Marley in front of the house (32:07).)
Just before Marv and Harry step into the small pile of toy cars and slip up at the bottom of the stairs, Harry had been covered in feathers. In the next shot, he has no feathers on him at all, they just mysteriously disappear.
When Harry and Marv follow Kevin in their van, they initially stop when they are slanted on the road. After the stop for when Kevin turns around to look at them, they are fixed perfectly against the curb.
When Kevin and Buzz are first seen fighting over the pizza Kate rushes over to break it up, in the next shot when Peter is throwing away the napkins Buzz is seen running his hands through his hair and Kate is talking calmly to Kevin, and in the next shot after Aunt Leslie asks Fuller if he's okay Kate is seen trying to break up the fight again and yells at Kevin.
Kevin purchases a large bottle of laundry detergent at the grocery store. While walking home, the two plastic grocery bags he is carrying burst spilling their contents. The bottle of detergent is not among the contents that spill.
When Harry and Marv are in the van and Harry is listing the automatic light cues, when Harry lists #671, the McAllisters' house, we can see the lights turn on in their reflection on the van's windshield. However in the next shot, which is of the house, we can see the lights turn on again.
When Kevin first uses the BB gun to shoot the figurines down the laundry chute, he pumps the rifle forward and then back at 23:21 on the DVD. Later in the film when he prepares to shoot the two robbers through the dog door, he pumps the rifle back and then forward at 76:12.
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.
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.
While setting traps in preparation for the burglars, Kevin douses all the stairs outside his house with water. They freeze solid in a short time, indicating it to be quite cold. However, later when we see Kevin running across the Murphy's driveway, not only are there visible liquid puddles, but he runs at a full sprint without so much as a slip.
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.
The plane in the movie is a McDonnell-Douglas DC-10-10, which was flown by American Airlines, however it was only used for domestic flights due to its limited range of 3500 miles. There was a long range version used for intercontinental flights, the DC-10-30, but that one had three sets of rear landing gear. It is clearly visible during the takeoff scene that this one only has two sets of rear landing gear, so it is definitely a DC-10-10 which could have never made the trip from Chicago to Paris.
When the alarm clock turns off because of the power loss, we can clearly see that this Panasonic clock has a battery back-up feature, which means that even after the blackout, it shouldn't have stopped working.
When Kevin makes the sign of the cross before eating his dinner, he makes it what would appear as wrong, touching his right shoulder first - however, some eastern catholics, such as Byzantines, bless themselves this way.
When the pizzas arrive, the guy says the cost will be $122.50. In the kitchen, however, Peter says "Ten pizzas times twelve bucks.", which would mean the pizzas should cost $120.00. For the pizzas to cost $122.50, they would need to cost $12.25 each. However, the extra $2.50 was most likely added as a delivery charge, or an automatic gratuity considering the size of the order.
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. The McCallisters are calling the police in their home village of Winnetka, which is a Chicago suburb.
In the beginning of the movie after Peter realizes he will not be able to shave in Paris without a power adapter, Kate tells him to grow a goatee. This would be impossible, since a goatee would still require shaving part of your beard. However, it seems like Kate is merely making a sarcastic joke while Kevin is bugging her.
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.
When Kevin is at the counter of the grocery store, the clerk scans a bag of toy soldiers and looks at Kevin. He says "It's for the kids". But when the clerk asks, "What about your brothers and sisters?" Kevin replies, "I'm an only child." While this seems that Kevin is stumbling over his lies, he is not. He is at the grocery store alone, buying "grown up" things such as orange juice, milk, bread, toilet paper and laundry detergent. He is also reading a tabloid and discussing the quality of microwave dinners just as an adult does. When the cashier scans the toy soldiers she looks at him questioningly. Kevin makes a joke saying they are "for the kids" referring to his own children. Obviously he doesn't have children, but his purchases and actions make him seem grown up.
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 Strassman 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 Kevin calls 9-1-1 to report that robbers are in his house, he uses a phone that says "Pacific Bell," which isn't available in Chicago. They would not receive service from Pacific Bell, but Pacific Bell does sell phones which can be used anywhere with any service.
On the phone with the French operator, Peter yells "I am looking for my son! Do you know where he is?" It seems a silly question as Peter should know that Kevin is at the house, but in a previous scene a police officer goes to the McAllister house and receives no response to repeated knocks and bell-ringings. It's likely that the police would've relayed this information back to the McAllisters and that this would explain Peter's frantic mood.
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.
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 kid's mind.
When Kevin is sliding down the steps the toboggan is lined-up with the stairway, when he goes out the door, the toboggan is now lined-up with the doorway. He should have just crashed into the wall and not gone out the door.
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 and we clearly hear an operator asking Kate to insert coins.
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.
When Kevin is shooting the toys by the rifle into the laundry chute, you can see that the third bullet doesn't hit the mark (the toy) but clearly passes above it (can be seen as a small dot of somewhat metallic color) and yet the toy falls down the chute.
When Marv is hit in the face with the iron, if you look closely before he falls down you can see that the iron hits him on the forehead, but when he sits up, the iron has left a print that covers his whole face.
In the film that Kevin is watching, "Angels With Filthy Souls", Johnny shoots Snakes with a Thompson machine gun. Snakes is standing in front of a door with a glass window. At the short range that Johnny shoots him, at least some of the bullets would have gone right through Snakes and broken the window.