At the end of the third movie, the spies catch Alex's chicken pox. Chicken pox can be lethal for adults.
Chicken Pox is very contagious for about 8 days (2 before the spots show and around 6 after). His siblings were still around him. Even if they've had it before, could they maybe have given it to kids at their school?
Near the end of the second movie, Kevin escapes his uncle's house by dangling a rope over the edge of the roof, climbing down said rope, and then lighting it on fire when Harry and Marv try to climb down after him, causing them to crash through the scaffolding and get covered in varnish. Unless one of the neighbors immeditely noticed the fire and called the police, that fire is going to spread to the house itself and, since it's a townhouse, probably to the other residences. And if the neighbors are at home in bed asleep...
Unless they had smoke detectors (though a housefire on Christmas is still an upsetting thought).
It seems like a bad neighbourhood; most of the houses look empty. Plus, the paint and the fall might have put the fire out. Harry and Marv weren't set alight when the rope fell, after all.
The blowtorch gag from the first movie. Thank goodness Harry was the one to go through that door, and he just got the top of his head burned. If it had been the much taller Marv, that would have been his face.
In the second movie after defeating Harry and Marv, where was Kevin planning on spending the night? He obviously couldn't have known his mom would find him in Rockefeller Center, he couldn't go back to the Plaza Hotel or any hotel seeing as his dad's card was put on hold, and probably thought he was wanted by the cops. It was below freezing that Christmas Eve (remember the frozen puddle), and he didn't exactly look overdressed. Plus all the few warm spots like subway gratings and vents would be occupied by potentially mentally ill homeless people. "It's scary out there", indeed.
Being an 8-year old and fearing the burglars who are gonna break into your house.
He also shoplifted a toothbrush and, being a kid, might think he's still in deep trouble over it.
They say near the beginning that the tree that ruined the power lines and made them late also ruined the phone lines. And all the neighbors are gone for vacation. Why Kevin doesn't go to the store or the church that he is apparently in walking distance of is anybody's guess.
In the first film, Kevin clearly sees Harry dressed as a police officer, that may have played a role.
Kate didn't really need the Polka King: why couldn't she just rent her own car and drive herself?
Because she gave all her money and jewelry to that old couple in exchange for their plane ticket, and she was exhausted from traveling. Plus, renting a car would mean returning it, so she couldn't do that either.
In addition debit cards do not exist and only few high end places would accept credit cards. I suspect that interstate car rentals were very rare too since Onstar, GPS, or any tracking system was non-existent.
On another note, New York in the second movie somehow becomes completely deserted, but only when Kevin faces off against Harry and Marv (the rest of the film correctly shows crowds of people). And this is the "City That Never Sleeps"!
In the first movie Kevin's 8 and in the second movie he's 10, meaning it's two years after the first. So why is everyone referring to the events of the first film as last year?
That may have been a last minute script change, or an oversight.
It does work if the first movie was just before his birthday, and the second just after.
It is the same general time of the year, but if Kevin's birthday was on a fixed calendrical date after the family left the first year and before they left in the second movie, it works.
The only evidence we have of his age in the second film is his word; Kevin may have altered it himself, for whatever reason.
Harry and Marv seem to get dumber and dumber to the point of Flanderization. Then again they have had a LOT of head injuries...
Harry: "Don't you remember what happened last year?"
Marv is the only one who seems to get dumber between the two films. Harry actually anticipates some of the traps in the second film, though he doesn't count on them being modified.
In Home Alone 3—how sick was the kid? We never see anything more than spots on him and yet he has chicken pox. He should at least have occasionally scratched a bit, but he's running around the house like he's healthy on the fifth day. Yes, technically antibiotics can make you feel okay while still contagious, but there was no indication that he was sick at all, which is weird since it's the whole reason he was home alone. I'm not asking for debilitating or anything, but a reminder or two furing the film of "oh, right, he's sort of got one of the worse diseases a kid can have" may have been nice.
He is itchy though. Mrs. Hess calls him out on "scratching in front of a lady", his parents tell him not to scratch, and his brother makes a joke about the resulting scars. They probably, off-screen, slathered him in calamine lotion and had him take oatmeal baths (which really cuts down on the itching). Personal Anecdote
My sister and myself both went through chicken pox (two seperate incidents at 9 and 5 respectively) and it barely slowed us down, itchy but that's about it. My mother, on the other hand, at 30-something was bedridden.
(Putting it here before it comes up:) The dog door on the the backdoor of the Mc Callister's home. They have it because they have a dog (that we never see), Kate mentions that it's in the kennel when she's on the phone at the very beginning of the movie (just before telling Kevin to get off the bed).
Of course that brings up the question (as any pet owner would notice) where is the rest of the dog's things (toys, etc)? They wouldn't have brought it all to the kennel. Also, it's never mentioned in the second film.
Some families keep all their dog's toys in a basket when the dog's not using them (so no one trips on them), and train the dog to just get them out of the basket if they want anything. If the dog's not there, they may have put the basket in a closet or something.
Given how crowded the house was with guests, and the possibility one of those guests might have been allergic or just didn't like dogs, they might've exiled the family pet to the backyard for the few days prior to their flight. Putting its toys outside with it would help lessen the sting of being evicted.
They could also put the dog in a Kennel because most flights don't allow dogs.
Actually, given how many relatives they had to deal with, they probably had an unusual attack of practicality and put the dog into kennels a day or two before they left to make preparations smoother. Flying to France with a largish dog is not terribly easy at the best of times.