Dustin's first appearance in Season 3 has the rest of the group pranking him with his toys apparently coming to life, in what was likely a deliberate nod to Gaten Matarazzo's upcoming role hosting Prank Encounters, about people coming in for a day's work only to be pranked by something seemingly supernatural. Trouble is, by the time the season was released, that show had suffered an astonishingly poorly worded announcement that gave the impression it was preying on people looking for long-term jobs, forcing both Netflix and Gaten himself to clear things up.
The first episode of Season 3 focused on Hopper trying to give Eleven a "heart-to-heart" talk about her relationship with Mike, only to be playfully joked at by Mike and Eleven. This becomes incredibly more saddening considering the final episode of the season reveals that the talk was less about Hopper wanting Eleven to set boundaries on her relationship with Mike and more about him just wanting her to still be his daughter despite growing up. Ouch.
It's not 100% clear whether Hopper writes his own, heartfelt addition to Joyce's touchy-feely, very un-Hopper-like script before or after his aborted attempt to deliver that script. The Hopper who lies to and bullies Mike is kind of hard to square with the Hopper in the voice-over at the end of the season, suggesting that maybe his additions were written at a later point when he had calmed down and was still hoping for a good opportunity to have that conversation (maybe with El alone, rather than El and Mike together).
Dustin telling Hopper that they could defeat the Mind Flayer with an army of zombies was played as a joke in Season 2. Then in Season 3 we see the Mind Flayer's brainwashed minions ingesting toxins, decomposing and turning into horrific monsters. Just like zombies.
When he first sees Eleven, Dustin asks whether she has cancer. In the season finale, we find out that Hopper's daughter died of it.
Nancy's justified anger towards her mother regarding her lack of focus on Barb's disappearance and more onto the fact she slept with Steve becomes eerily uncomfortable when the final two episodes focused more on Will's rescue from the Upside Down and less on Barb's death. The fan reactions made it worse, to the point that Season 2 kinda had to work to resolve this.
Karen, who had upbraided Nancy for having sex, came close to cheating on her husband with Billy.
Billy taking his lifeguard job seriously becomes quite poignant upon finding out that his childhood dream was to be a surfer.
In "Vecna's Curse", Mike treats Will like a third wheel during his date with Eleven and blames him when it goes badly, accusing Will of moping and acting depressed. This becomes a lot harsher when fans noticed that the episode takes place on Will's birthday, which seems to be completely ignored by his friends, which the Duffers admitted they didn't catch.
Mike encouraging/pressuring Will to say yes to the girl who asks him to dance at the Snow Ball in Season 2 is quite hard to watch after Season 4 implies that Will is gay and in love with Mike.
Will wanting to continue playing D&D with his friends in Season 3 is portrayed as him clinging onto childhood while his friends are moving on, and could even be interpreted as him struggling with his sexuality. Will giving his D&D paraphernalia to Erica and warmly telling Mike he'll never want to play with another group is portrayed as Character Development and him accepting growing up. Except... in Season 4, Mike, Dustin, and Lucas have all started playing D&D again with the Hellfire Club, the game is portrayed far more maturely (to the point they're reluctant to let eleven-year-old Erica play with them), and Will is now the only one from their original group who doesn't play at all.
Season 4 contains some prominent product placement for Jif peanut butter, with Yuri being a proud smuggler of Jif products and Hopper nourishing himself on a stash after his first escape from prison. Just a week before the premiere of Season 4's first volume, the J.M. Smucker Company had issued a massive recall of Jif products over potential salmonella contamination, and the products were still largely unavailable at the time of Volume II's premiere.
In the penultimate episode of Season 2, Dustin compares the Mind Flayer to the Nazis, alikening its desire to lord over humans to Hitler's desire to see a "master race" of humans. The reveal of what the Mind Flayer (and by extension, the being controlling it) really is shows that Dustin was right on the mark.
Max escaping from Vecna to a triumphant musical score becoming the Signature Scene of the series can feel a bit less triumphant when he puts her in a coma during a subsequent encounter a few episodes later.
, and the products were still largely unavailable at the time of Volume II's premiere.