Harry Potter is peppered with Foreshadowing and Chekhovs Guns throughout all seven books, so things which aren't clear or alluded to with the first read will stand out in subsequent readings.
- One example is Sirius Black being mentioned in passing in the first chapter of the first book when he wouldn't be introduced until the third.
- Sirius's actions during Book Three make a lot more sense when you already know that he's innocent of the crimes he supposedly committed, and several of them actually serve as foreshadowing.
- It's hard not to read Hagrid's proclamation to Harry that "every wizard who went bad was in Slytherin" without thinking of Peter Pettigrew, who was a Gryffindor.
- It's quite funny to read through the books and see how wrong the main characters get everything. Like when Harry hears Snape's and Quirrell's conversation in the Forbidden Forest in Harry Potter and the Philosopher's Stone.
- Book 7 adds another layer to Snape and Quirrell's conversation, since you find out he was extra-vigilant on Dumbledore's orders in "The Prince's Tale":Dumbledore: Keep an eye on Quirrell, won't you?
- Also, Snape's muttering the countercurse while Quirrell hexes Harry's broom makes way more sense after book 7, when you learn that Snape is still protecting Harry out of his love for Lily, Harry's dead mother.
- Book 7 adds another layer to Snape and Quirrell's conversation, since you find out he was extra-vigilant on Dumbledore's orders in "The Prince's Tale":
- It is first mentioned off-hand that Nick got Peeves to drop the Vanishing Cabinet and break it, in order to distract Filch from Harry. In the second book, Harry hides in a cabinet in Borgin and Burkes' to avoid meeting Malfoy and his father. In book five, Fred and George say they shoved one of the Slytherins into the broken Vanishing Cabinet. Well, in book six, the Vanishing Cabinet plays a huge role. Malfoy realized that the two vanishing cabinets form a passage between each other, he's been spending a lot of time in the Room of Requirement to fix it and it's also how Death Eaters got into Hogwarts at the end of the book, despite the stricter safety measures!
- Dumbledore’s hatred of Azkaban in general and dementors more specifically makes more sense knowing that his dad died in Azkaban.
- Dumbledore's "brief glint of triumph" in Book 4 after Harry said Voldemort took his blood to revive himself. He figured out Voldemort himself unknowingly gave Harry the best protection from any attempts he can make, as it prevented Voldemort from killing him without killing himself.
- In the first book, during Harry and Ron's first meeting with Malfoy, when Malfoy insults their parents, Scabbers the rat proceeds to bite Goyle's finger and manages to cause Malfoy, Crabbe and Goyle to leave. After the reveal that Scabbers is actually Peter Pettigrew, Pettigrew took Malfoy's insult about Harry's parents personal.
- A lot about book four, very early scenes make a lot more sense upon re-read. In particular, Winky sitting in the Top Box, her fleeing from the campsite, exactly how far ahead Crouch Jr. planned by giving the water plant book to Neville after their first lesson, his overall help to Harry during the tournament and Voldemort's speech of the faithful Death Eater residing in Hogwarts. Also, Moody's surprise when getting a look at the Marauder's Map, and him confiscating the map from Harry, makes much more sense when the reader discovers Moody's true identity. The map nearly ruined Fake Moody's whole plan and would have ruined his plan at that very moment if Harry had gotten a look at it.
- In book one, Harry gets the feeling that Snape can read his mind. In book five, Dumbledore assigns Snape to teach him how to stop people from, basically, reading his mind, being very good at both reading minds and preventing his mind from being read.
- Rewatch all the times before Book 5 where Harry did not block making eye-contact from Snape and wondered how Snape knew that tidbit of information.
- In Book 4, at one point Snape accused Harry of stealing the ingredients for a Polyjuice potion (actually Crouch Jr.). Snape never accused Harry again after that, because he actually sees in Harry's mind that he and his friends aren't the actual thieves this time (Harry and Co. did really steal the ingredients to make a Polyjuice potion, but that was 2 books earlier), and, in fact, know nothing about the current heist.
- Early in book five, Petunia reveals she knows what dementors are because she heard "that awful boy" tell Lily. Harry and first-time readers assume she's talking about Harry's father, James. Re-readers might recognize that she actually meant Snape.
- On a re-read, it’s clear that Dumbledore’s brother Aberforth, who owns a bar in Hogsmeade, feeds him a lot of information. He tells Riddle in a flashback in the sixth book that he knew he was staying there because he was friends with the local barman. It’s also how he finds out about the defense club in the fifth book. Harry even notes that Aberforth looks vaguely familiar when they’re about have their meeting but can’t put his finger on why it is he looks familiar.
- Snape's mutual hatred of Sirius seems to be solely based on Sirius bullying Snape in their teens, hence a lot of characters wondering why he can't simply let it go. Re-read book 3 with the knowledge that everything Snape did was in revenge for Lily's death since everyone thought it was Sirius who betrayed the Potters. Even if Snape learned Pettigrew was the traitor, it was still Sirius' idea to make Pettigrew the Secret Keeper, and in a way his fault they were murdered.
- Similarly, pay attention to Snape's treatment of Pettigrew in the sixth book.
- In the first book, Butt-Monkey Neville Longbottom gets into a fight with Crabbe and Goyle to keep them from ganging up on Ron (who is fighting Draco). Ron mentions that Neville is in the hospital wing, but has no serious injuries himself. This was the first indication that Neville, meek as he was, was placed in Gryffindor for good reason and his future role as a protector of others, but the first time you read it, it just seems like another joke at Neville's expense.
- One of the reasons why Neville joined the fight in the first place is because Malfoy made a rude ableist crack about the mental ward in St. Mungo's. The Reveal that his parents are permanent residents there because they were tortured into insanity by the Death Eaters puts Neville's reaction to what Malfoy said in a whole new (and heartbreaking) light.
- In something of a montage sequence in Book 5, it’s offhandedly mentioned that the characters find, among other junk, "a heavy locket none of them could open". Re-reading after learning about the Horcruxes in Books 6 & 7 cause the reader to realize that they had a Horcrux in their possession the whole time.
- In a similar vein, when Harry stashes his Potions book in the Room of Requirement in Book 6, it's mentioned that he uses a bust to mark where he left it, putting a dusty wig and a 'tarnished tiara' onto it. The last one turns out to be another Horcrux found unwittingly.
- In book 6, Dumbledore's final words are heartbreaking in two very different ways. On the first reading, you believe that Dumbledore, shocked by Snape's betrayal, is begging him not to kill him. However, on the second reading, you realize it's actually the exact opposite, Dumbledore is begging Snape to go through with their agreement to kill him, so as to gain Voldemort's trust. In addition, the look of "hatred and revulsion" on Snape's face, seemingly directed at Dumbledore, is actually directed at himself.
- Related to the above, Hagrid tells Harry that he accidentally overheard Dumbledore and Snape having an argument, and that Snape sounded particularly angry and suggested that he might have "changed his mind". Come the final book, they were arguing about the aforementioned Mercy Kill Arrangement, which was the cause of Snape's anger in the first place.
- In Harry Potter and the Chamber of Secrets, anytime there was an attack, the narration made sure to include Ginny's reaction. Many little moments also mentioning Ginny throughout the story makes a lot more sense during a second read.
- After Mrs Noris was attacked, Ginny was mentioned being disturbed. Ron mentioned that she was a cat lover.
- After Colin Creevy was attacked, Ginny who had sat next to Colin in class was distraught.
- After the double attack on Nearly Headless Nick and Justin, Fred and George joke about Harry being the culprit. There was a mention that Ginny didn't find it amusing.
- Way back in chapter 5, the third reason why the Weasleys had to turn back home was because Ginny nearly left her diary behind.
- When Ginny noticed Harry has the diary in his bag, Ginny stared between Harry and the diary with terror.
- When the trio visit Hagrid one time, Hagrid offhandedly mentioned he had seen Ginny roaming the grounds, presumably looking for Harry. We later find out Ginny was looking for roosters to kill.
