When the girls make peace, it's Annie that makes the first move by helping Hallie close the window. Hallie was more openly hostile and instigated the fights between the two. It would make sense that Annie would be the one to be helpful as Hallie had more contempt towards Annie than Annie did towards Hallie.
Both girls liking Oreos and peanut butter seems like a cute and random way to show the girls are alike. Similar twin taste buds, something the parents used to eat when they were married to each other, or a residual preference from a wacky craving Elizabeth had when she was pregnant with them? Considering the rarity of both Oreos and peanut butter in the UK, it's likely that Nick introduced the combination to Elizabeth.
Bordering on Fridge Logic: When Meredith gives her ultimatum to Nick to choose between "[her] or them", she brings up that she plans to send both twins to boarding school. The thing is, only Hallie belongs with Nick. Annie's the twin who belongs with Elizabeth. For Meredith to be so presumptuous to assume she has a say in what happens with both Nick and Elizabeth's daughters must've been just as much a red flag to Nick as saying she would essentially ship off his flesh and blood. Either way you look at it, Nick had more than enough reason to choose his daughters over Meredith.
Also, Nick just got his other daughter back, and is clearly heartbroken in the later scene about having to give her up. While he and Elizabeth have discussed the idea of splitting them up, there's no doubt he would have done his best to keep in touch with Annie, see her on vacations, be there for her when she needs him, etc. The idea of sending her off to boarding school is just as much a red flag as Meredith threatening anything happening to Hallie.
Annie doesn't yet know that the father Hallie starts raving about to her is both of theirs, so once she starts to do so, Annie retreats to her bed not because it's chilly in the room, but because hearing Hallie jabber on about her father when (she thinks) she doesn't have one is making her very uncomfortable.
Also, this is immediately after Hallie showed her a photo of her (their) dad. We don't know what angle the picture is taken from or how long ago it was taken, and of course people change a lot over time, but is it possible she, at least subconsciously, recognized him from her half of the photo? That would also explain her disquiet, if she's wrestling with how much Hallie's dad looks like hers.
The parents' original wedding montage is set to Nat King Cole's "L.O.V.E." Their second wedding in the finale is set to the song "This Will Be (An Everlasting Love)," sung by Natalie Cole, who was Nat King Cole's daughter. It serves as a nice way to symbolize new beginnings and tie the two together.
The sound bite played when Hallie and Martin do the secret handshake is the intro of "Soulful Strut" by Young-Holt Unlimited, which is an instrumental remake of "Am I the Same Girl" by Dusty Springfield (originally Barbara Acklin). Further, the lyrics of the original song "Why don't you stop and look me over? / Am I the same girl you used to know?”, are definitely invoked even without being present.
Both Hallie-as-Annie and Annie-as-Hallie occasionally slip up and use English or American words or terms when they should use the opposite. One noticeable moment is when Hallie-as-Annie offers to 'mail' Cuppy back to Annie-as-Hallie; the vast majority of people raised in England would say 'post' instead. Likewise, Hallie says "closet", when in England they'd probably say "wardrobe" or "cupboard".
Annie slips up less when pretending to be Hallie, only the expressions like "lovely girl", "horrid habit" and "gave me a fright", and the latter only comes out when she's under stress. Because she spent eight weeks at Camp Walden, surrounded by American girls and therefore would pick up the slang much more. Which, incidentally, is why any slips in slang that Hallie made would be overlooked (until the Cuppy scene, anyway).
Meredith says she "hates things that crawl", ostensibly as a reference to lizards and creepy crawlies. But babies crawl as well. Fitting since she's a Child Hater.
Lizzie sends Martin to pick Annie up at the airport rather than going herself. Natasha Richardson suggested that this was because Lizzie was baking a cake for her daughter (that they'd then share in the Cuppy scene). But Martin is very close with Annie too - and she's bound to know about their secret handshake. It's possible she felt Martin would like some alone time with Annie after she'd been gone for eight weeks too.
In the ear-piercing scene, Hallie sterilizes the needle with a match. Which means that the adults at the camp were so inattentive that they allowed an eleven-year-old to get her hands on and light a match. To add to the fridge horror, the isolation cabin was in a wooded area meaning that if Hallie hadn't been as careful with the match, she could've caused a forest fire in addition to burning down the cabin.
The scene also demonstrates that no one will hear two preteen girls screaming at the top of their lungs. A rapist or ax murderer or wild animal could have gone after them, and the adults responsible for their safety would have slept right through the whole thing because the Marvas thought it was a bright idea to punish two misbehaving children by putting them too far away to properly supervise them.
Pictures torn in half, no mention of your other parent, meeting a lookalike at camp, being shipped halfway across the world—only to discover you've been lied to by EVERYONE. Imagine the therapy sessions (if the movies weren't Disney, that is).
Interestingly, for all the Disneyfication American films underwent compared to the original book and its original German adaptation, they became much more nightmarish in this regard: in the original, nobody at all except the parents themselves knew of the twins (the workers of the summer camp guessed the truth but decided they were not the ones to disclose it), the camp was closer to the twins' homes, and the girls themselves came to terms with the whole situation pretty soon (in fact, for all the mutual vicious pranking in American adaptations, there's just a single unanswered kick in the original).
Try watching this movie in a post-9/11 world. The girls are presumably able to switch passports and tickets to fly as each other to London or California without a hitch. Think of the implications now; how easy would it be for twins to abuse this idea at airports for less innocent purposes?
Or more likely be found out before even meeting their respective parents.
When the father first sees his ex-wife, his first question is along the lines of "WTF are YOU doing here?" Not even a thought about his lost daughter until she appears before him.
Presumably that would have been the next question if the girls had not shown up.
No indication that the conversation was going in that direction. He spoke about his wedding.
He probably just assumed that she was still in London, and the ex was there for some other reason.
It always seemed to me that he was simply so shocked and overwhelmed by seeing Elizabeth again that he hadn't had a chance to process that before the other twin showed up. Hell when he sees her a second time he falls into the pool. This is his ex-wife who he hasn't seen in 10 years and is quite obviously still in love with... showing up out of the blue only weeks before he's going to be getting married to another woman. Plus it's Natasha Richardson. In his position I'd probably be thinking 'holy shit she looks phenomenal how did I ever let her go' rather than 'holy shit she's here so maybe Annie is too'
Who's the judge who signed off on this custody arrangement and what other horrors are they perpetrating upon the defenseless children of dysfunctional couples?
While the onus of the Fridge Horror is definitely on Nick and Elizabeth, Chessy, Martin, and the girls' grandfather were all in the know about the arrangement, but went along with the lie for over a decade...because reasons? None of them thought to speak up, none of them were conflicted over having to pretend the other twin didn't exist, none of them thought to say "hey, perhaps this no-contact thing with the other parent is psychologically damaging to your kid"? Why is the grandfather a-okay with not seeing one of his grandchildren ever again? Chessy was nanny to both girls prior to the divorce and knew Elizabeth (whom Hallie doesn't remember, but wishes she knew) and she's also okay with this arrangement and lying to Hallie? What is wrong with this family?!
The Doylist answer is that in the 90s, screenwriters didn't really think of these implications because we didn't have the internet or TV Tropes to discuss them. Media suddenly became a lot more realistic or the illusion of realism in the 2000s (1999's Fight Club has a bunch of skyscrapers being destroyed as an unambiguously happy ending). The Watsonian one - Nick and Elizabeth were hot tempered and irrational in their youth. If you do the math with the twins' ages, and if the parents are the same ages as their actors, Elizabeth was 22 when they got together. A Freeze-Frame Bonus says that Nick is ten years younger than Dennis Quaid, so he was 23-24 when it all happened. It seems to have been a youthful whirlwind romance that they thought was love, and they hurriedly got together but then they have to deal with two babies at once and the fact that they barely knew each other and were expected to build a life together at such young ages - especially from different countries too (the original script said they tried living in both places, which can't have been easy). The split from the sounds of it was very nasty and the arrangement seems to have been a reason to never see the other person again. They may have felt splitting the twins would be a weird kind of compromise (how exactly do you share custody when the parents are on different continents?); maybe they did initially keep in touch or plan to when the girls were older, but it was too painful or never seemed like the right time to do so. It seems as though they let their careers take priority as well. So it might not have been done out of maliciousness, but a snap solution that ended up escalating and their only way of dealing with it was ignoring it altogether.
Meredith could have easily rolled off of the air mattress and fell in the lake and drowned. People can move around a lot in their sleep and if she fell in the water, it wouldn't be much of a stretch for her to be too disoriented to figure out what was going on or be able to figure out which way was up. Hallie and Annie are seriously lucky.
Which is why they are "punished through the end of the century".
Not to mention that both twins heard Meredith say that she had taken a sleeping pill before going to bed, one of them even mentions it while they're dragging her out of the tent.
