1.
The beginning of the film has many setups for Kevin's traps later. For example, Mr. McCallister tells Kevin to take his micromachines because “Aunt Leslie stepped on one of them and almost broke her neck.” Which, of course, is exactly what happens to Harry and Marv later in the film.
2.
Kevin's mom then tells his dad that Kevin was “playing in the garage with the glue gun again,” who sets up the glue gun* that Kevin later uses to cover the plastic wrap.
3.
Kevin explains that he was simply “making jewelry out of fishhooks.” Later, we see Kevin placing these decorations on the floor under the window, and of course, poor Marv steps on them… barefoot.
4.
Apparently everyone is waiting for Kevin to burn the house down. When Kevin's father reprimands him for using a glue gun, Kevin responds, “Did I burn a joint? I don't think so”. And at the end of the film, when his family returns, Buzz tells Kevin, “It’s great that you didn’t burn this place down.” I CAN'T LAUGH.
5.
Here in the background sits a stuffed figure of Uncle Frank, sitting alone and doing nothing.
6.
Little Nero's Pizza slogan: “No fuss!” which is funny because of the expression: “Nero tinkered while Rome burned.” Essentially, this saying means doing something trivial and irresponsible during an emergency. Come to think of it, that's what Kevin does for most of the movie!
7.
You probably didn't notice this the first, second, or even fifth time you watched the movie, but if you actually listen (or watch with subtitles), you'll notice that when Uncle Frank says he has no cash, only traveler's checks, Kevin's father mutters, “You probably have those traveler's checks that don't work in France“
8.
Kevin's plane ticket and passport are accidentally thrown away after the spilled milk incident, which explains why no one initially notices that Kevin is not on the plane.
9.
There's a random pair of roller skates in the kitchen, which only becomes funny when you notice them later appearing in the basement and then again in the kitchen.
10.
The Family Crisis Intervention officer who talks to Kevin's mom is called Sergeant Balzac…a funny name that probably crossed your mind as a kid.
11.
Kevin can't find his toothbrush because his mom packed it for a trip.
12.
It's a little hard to hear, but if you really listen, you'll notice that Herb, the pharmacy employee who asked if he knew “if a toothbrush is approved by the American Dental Association” (and didn't), responded cheerfully with DGAF because he said to another employee, “Why don't you just tell him it's approved?”
13.
The address Kevin gives Santa, “671 Lincoln Boulevard”, is actually the real address of the house used for the exterior of the McCallister house in the film (essentially).
14.
The guy next door who is confusing the staff numbers is Mitch Murphy. This is his family's home, which we see Harry and Marv rob later in the film, and where Kevin runs (when he is caught by the Wet Bandits) near the end of the film.
15.
Among more obvious things like the firecrackers he uses later in the film, Kevin also steals Buzz's Crunch Tators from his chest. Later they sit on his side table while he eats his huge bowl of ice cream.
16.
Speaking of Buzz's things, you can see the basketball action figures that Kevin knocks down the garbage chute later in the film on Buzz's bookshelves. I guess this means Kevin brought the action figures from the basement and put them back on Buzz's shelves (before smashing them, lol).
17.
And it's really random, but funny, Buzz has some kind of Indiana Jones parody card on the note board above his desk. (Looks like it says “Jimmy” lol?)
18.
It's hard to say, but to make his Little Nero pizza delivery ~trick~ more effective, you can see that Kevin brought HUGE speakers into the kitchen.
19.
A red Christmas bow that magically flies away during a thunderstorm reappears seconds later on a lamppost.
20.
Like Kevin's dad said, the garage door actually stays open, but it's STRANGE that the cars don't even drive all the way into the garage.
21.
The “robe” that Kevin uses to disguise himself in the nativity scene is actually just a display curtain…
22.
…and someone at the church must have fixed it, because when Kevin returns later, it freezes again.
23.
Kevin carved a basketball out of Michael Jordan poster in Buzz's room, and he had to tape Jordan's arm together…
…although ~magically~ the poster reappears, basketball and all, in Buzz's room later in the film.
24.
Several of the mannequins that Kevin uses for his impromptu ~party~ can be seen in the basement earlier in the film.
25.
Also, if you look closely at this mannequin's hand, you'll notice a Coca-Cola can…
26.
…And it's funny/ironic because Pepsi features prominently in the movie (“Fuller, go easy on the Pepsi!”). Apparently, someone in the McCallister family prefers Coca-Cola.
27.
You may see what looks like tiny pebbles (or maybe grains of salt?!) being thrown at the spider to get it to move around in the bathtub.
28.
When Kevin's grocery bags break, a large container of Tide detergent goes missing.
29.
And if you're always wondering, the bag tearing gag was achieved using rope, which you can see if you look closely.
30.
It's not easy to hear, but if you listen closely, the furnace in the basement really says, “Hey, Kevin.”
31.
Kevin's mac and cheese dinner seems very different. frozen dinner a few moments later when Harry enters the dining room.
32.
If you're wondering why there's a dog door in the house but no dogs appear in the film, it's because the McCallisters put the dog in a kennel for the duration of the trip.
33.
And finally… apparently Harry is married.
