J.K. Rowling is the queen of nuance. Sometimes even I am surprised at what she came up with. Below is my list of the best bits of foreshadowing and cool pieces of information from the Harry Potter series.
Note: There will be spoilers for those who have not finished the books.
1) Gringotts
"…yeh'd be mad ter try an' rob it."
In "Sorcerer's/Philosopher's Stone" there were many details about Diagon Alley that J.K. Rowling could have incorporated, but for some reason this dialogue between Harry and Hagrid was vitally important. Why? Because if we fast-forward to "The Deathly Hallows," Harry and his friends, Ron and Hermione, do break into Gringotts and free a dragon.
2) Ollivander
“The wand chooses the wizard, Mr. Potter. It's not always clear why."
This piece of information from "Sorcerer's/Philosopher's Stone" is important because while we know Harry and Voldemort's wands are brother wands, we assume Ollivander is just telling us all this because he loves wands and wandlore. In reality, we learn near the end of "Goblet of Fire" that because of their “twin" status, Harry and Voldemort's wands have a special connection. We also come to realize in "Deathly Hallows" that the wand choosing the wizard has so much more to do with everything and how being the “master" of a wand has such an importance.
3) Albus Dumbledore
"There are all kinds of courage," said Dumbledore, smiling. "It takes a great deal of bravery to stand up to our enemies, but just as much to stand up to our friends. I therefore award ten points to Mr. Neville Longbottom."
Dumbledore says this in "Sorcerer's/Philosopher's Stone" at the end of term feast. We also see throughout this book that Dumbledore's Chocolate Frog Card, which mentions his fight with Gellert Grindlewald, has made multiple appearances. These are both important foreshadowing of Dumbledore's story because in "Deathly Hallows," we learn that it took a lot of courage for Dumbledore to kill Grindlewald. He understands Neville's position much more than we realize when it happens in "Sorcerer's/Philosopher's Stone!"
4) Severus Snape
Harry, who was starting to feel warm and sleepy, looked up at the High Table again. Hagrid was drinking deeply from his goblet. Professor McGonagall was talking to Professor Dumbledore. Professor Quirrell, in his absurd turban, was talking to a teacher with greasy black hair, a hooked nose, and sallow skin.
It happened very suddenly. The hook-nosed teacher looked past Quirrell's turban straight into Harry's eyes — and a sharp, hot pain shot across the scar on Harry's forehead.
“Ouch!" Harry clapped a hand to his head.
“What is it?" asked Percy.
“N-nothing."
The pain had gone as quickly as it had come. Harder to shake off was the feeling Harry had gotten from the teacher's look — a feeling that he didn't like Harry at all.
“Who's that teacher talking to Professor Quirrell?" he asked Percy.
“Oh, you know Quirrell already, do you? No wonder he's looking so nervous, that's Professor Snape. He teaches Potions, but he doesn't want to — everyone knows he's after Quirrell's job. Knows an awful lot about the Dark Arts, Snape."
Harry watched Snape for a while, but Snape didn't look at him again.
Now, this scene in "Sorcerer's/Philosopher's Stone" is a small bit of foreshadowing many people probably missed, but it's there if you look hard enough. Most people who meet Harry Potter look at the scar first, right? But, where does Snape look? Into Harry's eyes, his mother's eyes. Snape gave himself away as a good guy in the very first book by focusing not on the scar and the war, but on his love for Lily, and no one realized it. We do get confirmation of Snape's allegiances in "Deathly Hallows" when Harry sees Snape's memories after he dies.
5) Protection of Sorcerer's/ Philosopher's Stone
In "Sorcerer's/Philosopher's Stone,"there were seven levels of protection placed on the stone: Fluffy, the three-headed dog, borrowed from Hagrid; the Devil's Snare plant, from Professor Sprout's greenhouses; the flying keys and broomstick, enchanted by Professor Flitwick; the giant chess set, transfigured by Professor McGonagall; a larger troll, placed by Professor Quirrell; the potions riddle, placed by Professor Snape; and lastly, the Mirror of Erised, placed by Headmaster Dumbledore. So, how is this foreshadowing? Well, there are seven Harry Potter books, Voldemort makes seven Horcruxes, there are seven Weasley children, Harry paid seven Galleons for his wand, there are seven main classes at Hogwarts (Astronomy, Charms, Defence Against the Dark Arts, Herbology, History of Magic, Potions, and Transfiguration), and many more.
6) Bezoar
Snape's lips curled into a sneer.
“Tut, tut — fame clearly isn't everything."
He ignored Hermione's hand.
“Let's try again. Potter, where would you look if I told you to find me a bezoar?"
This was in "Sorcerer's/Philosopher's Stone" and makes me wonder if perhaps Snape was really teaching his students. Then in "The Half-Blood Prince," Harry gets Snape's old book and reads that to treat most poisons you can shove a bezoar down the person's throat. Later in "The Half-Blood Prince," Ron is poisoned and Harry can use the bezoar to save his life. J.K. Rowling foreshadowed the use of the bezoar way back in book one. We should have realized that someone was going to be poisoned and need a bezoar as an antidote, but we didn't realize it until the poisoning happened.
7) The Locket
They found an unpleasant-looking silver instrument, something like a many-legged pair of tweezers, which scuttled up Harry's arm like a spider when he picked it up, and attempted to puncture his skin. Sirius seized it and smashed it with a heavy book entitled Nature's Nobility: A Wizarding Genealogy. There was a musical box that emitted a faintly sinister, tinkling tune when wound, and they all found themselves becoming curiously weak and sleepy, until Ginny had the sense to slam the lid shut; a heavy locket that none of them could open; a number of ancient seals; and, in a dusty box, an Order of Merlin, First Class, that had been awarded to Sirius's grandfather for services to the Ministry.
In the "Order of the Pheonix," everyone is cleaning out the house to make it habitable for the Order. While doing so, they find a locket none of them can open. This locket, we later learn in "Deathly Hallows," is actually one of Voldemort's Horcruxes. Harry could have destroyed a piece of Voldemort way back in the fifth book if someone had made him aware of their existence and need for destruction, but alas, Kreacher stole the locket to protect it, then Mundungus stole it to sell it, and then it ended up in the hands of Dolores Umbridge (a whole other story).
