Everyone knows and loves Harry Potter, the book series written by J.K. Rowling. Rowling uses a specific type of writing style in her books called foreshadowing, which is a warning or indication of a future event. I know, I know. Just trust me on this. You'll see how awesome this will sound.
1. Locket They Couldn't Open
One of Voldemort's Horcruxes is Salazar Slytherin's locket. What people will usually not realize is that the locket was mentioned in Order of the Phoenix. Here's the situation: at one point, the characters are at 12 Grimmauld Place and they are looking through a bunch of stuff. Here's one of them:
"...also a heavy locket that none of them could open..."
This is something you're not going to notice unless you reread the books. But THAT'S Salazar Slytherin's locket.
Take a look at this quote from the very first Harry Potter book:
"The lake froze solid and the Weasley twins were punished for bewitching several snowballs so that they followed Quirrell around, bouncing off the back of his turban."
We later find out that Voldemort's face was on the back of Quirrell's head. So this means that they bewitched snowballs to bounce off the back of Voldemort's face.
3. Special Services
Here's a quote from Harry Potter and the Chamber of Secrets:
"I wouldn’t mind knowing how Riddle got an award for special services to Hogwarts either." "Could’ve been anything," said Ron. "Maybe he got thirty O.W.L.s or saved a teacher from the giant squid. Maybe he murdered Myrtle; that would’ve done everyone a favor…"
Well, we later find out that Tom Riddle actually DID kill Myrtle using the Basilisk and later used the murder to create his first Horcrux.
4. The Room of Requirement
In Harry Potter and the Order of the Phoenix, Harry and his friends discover the Room of Requirement, a hidden room at Hogwarts that will appear in front of you when you need something.
But Dumbledore actually mentioned discovering the room in Harry Potter and the Goblet of Fire. Specifically, at the Yule Ball.
"I took a wrong turn on the way to the bathroom and found myself in a beautifully proportioned room I had never seen before, containing a really rather magnificent collection of chamber pots. When I went back to investigate more closely, I discovered that the room had vanished. But I must keep an eye out for it. Possibly it is only accessible at five thirty in the morning. Or it may only appear at the quarter moon — or when the seeker has an exceptionally full bladder."
That was the Room of Requirement and it came to him when he needed to go to the bathroom.
Okay. I intentionally saved this one for last because this is my favorite use of foreshadowing in the Harry Potter books. This was a part in Harry Potter and the Goblet of Fire that was excluded from the film (they really took out a lot). But in this scene, Harry and Ron give up on their Divination homework and make up a bunch of stuff. And this is what they do.
“Okay… on Monday, I will be in danger of- er - burns.”
“Yeah, you will be,” said Ron darkly, “we’re seeing the skrewts again on Monday. Okay, Tuesday, I’ll… erm…”
“Lose a treasured possession,” said Harry, who was flicking through Unfogging the Future for ideas.
“Good one,” said Ron, copying it down. “Because of… erm… Mercury. Why don’t you get stabbed in the back by someone you thought was a friend?”
This actually foreshadows the Three Tasks of the Triwizard Tournament.
"On Monday, I will be in danger of-er-burns."
Harry is in danger of burns, actually. The first task involved him battling a dragon that can breathe FIRE.
"Lose a treasured possession"
In the second task, they have to retrieve a special possession in the Black Lake. Something that's treasured to the specific Champion.
"Why don’t you get stabbed in the back by someone you thought was a friend?
Now, this doesn't DIRECTLY foreshadow the third task. But it does have to do with something that takes place right AFTER the third task. The DADA teacher, Mad-Eye Moody, a friend of Harry, is revealed to be Death Eater Barty Crouch, Jr. in disguise.
I thought I was the only one that noticed that, but I found it online.