The Mirror of Erised: Erised is Desire spelled backward, or how it would appear in a mirror. Also, the inscription reads "Erised stra ehru oyt ube cafru oyt on wohsi," which read in a mirror reads, "I show not your face but your hearts desire." (Thanks, Kathryn)
They could hear footsteps, Filch running as fast as he could towards Peeves' shouts.
"Oh, move over," Hermione snarled. She grabbed Harry's wand, tapped the lock and whispered, "Alohomora!"
The lock clicked and the door swung open -- they piled through it, shut it quickly and pressed their ears against it, listening.
"Which way did they go Peeves?" Filch was saying. "Quick, tell me."
"Don't mess with me, Peeves, now where did they go?"
"Shan't say nothing if you don't say please," said Peeves in his annoying singsong voice.
Then, as you know, they turn around to face Fluffy who is glaring at them and about to attack. They were inside that room for quite a while listening to Peeves and Filch before they turned around to see Fluffy. Why didn't Fluffy attack as soon as they entered the room? Possibly because of Peeves' singsong voice, which might have started putting Fluffy to sleep. Fluffy didn't start baring his teeth until after Filch and Peeves had left.
Professor Quirrell, in his absurd turban, was talking to 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.
That sharp burst of pain did not come from Snape, but from Quirrell's turban, where Voldemort was "hiding." It should have been obvious after finishing the book, but for many of us, it wasn't!
After the students finish their exams and are lounging around by the lake, Harry gazes at an owl flying toward the school with a letter in its mouth. The letter that owl was carrying was the "urgent letter from the Ministry" that lured Dumbledore away from the school.
Turn this stupid, fat rat yellow!
Ron uses this spell in an attempt to turn Scabbers yellow. It doesn't work. Hermione wonders if it's even a real spell. But perhaps the reason the spell didn't work was because Scabbers wasn't actually a rat - it was Pettigrew in rat form. (Thanks, Holly!)
In the "enormous, old" reference book where Flamel is mentioned, it says that he was 665 years old as of "last year." That means he was the 666 (the Mark of the Beast) when the book was published.
Chamber of Secrets
In Borgin and Burkes: "Harry looked quickly around and spotted a large black cabinet to his left..." (pg. 50)
This is the same vanishing cabinet Draco uses to allow Death Eaters into Hogwarts in Half-Blood Prince.
In Borgin and Burkes: "'Can I have that?' interrupted Draco, pointing at the withered hand on its cushion. 'Ah, the Hand of Glory!' said Mr. Borgin, abandoning Mr. Malfoy's list and scurrying over to Draco. 'Insert a candle and it gives light only to the holder! Best friend of thieves and plunderers! Your son, has fine taste, sir.'" (pgs. 51-52)
Again, Draco uses the Hand of Glory in Half-Blood Prince.
In Borgin and Burkes: "Draco paused to examine a long coil of hangman's rope and to read, smirking, the card propped on a magnificent necklace of opals, Caution: Do Not Touch. Cursed -- Has Claimed the Lives of Nineteen Muggle Owners to Date." (pg. 52)
Yet again, Draco uses this cursed necklace in Half-Blood Prince. It is intended to be given to Dumbledore, but attacks the messenger, Katie Bell.
"In that case, perhaps we can return to my list," said Mr. Malfoy shortly. "I am in something of a hurry, Borgin, I have important business elsewhere today."
JKR has just told us Mr. Malfoy is doing something else today that may affect the story line: he ends up in Flourish and Blotts next and puts the diary into Ginny's cauldron. What was his business? And did it involve the diary somehow?
When Harry Ron and Hermione are trying to figure out what Tom Riddle did to get a trophy with his name on it:
"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..."
Of course, Tom Riddle (Voldemort) did kill Moaning Myrtle. (Thanks, Sarah!)
When Harry gets in trouble with Filch for tracking mud into the school, it is Nearly Headless Nick who saves him by encouraging Peeves to drop a cabinet on the floors above. It is very possible this is the same cabinet Draco repairs and uses to allow Death Eaters into Hogwarts in Half-Blood Prince.
Prisoner of Azkaban
When Harry and Hermione are using the Time Turner, JKR writes in extremely subtle things that occurred the first time around, such as Harry and Hermione having to hide in a closet from themselves. The Time Turner stuff is pretty complicated, here is where our Time Travel analysis can be found.
Chapter 11, The Firebolt: " GET-HIM-OUT-OF-HERE!" Ron bellowed as Crookshanks's claws ripped his pajamas and Scabbers attempted a wild escape over his shoulder. Ron seized Scabbers by the tail and aimed a misjudged kick at Crookshanks that hit the trunk at the end of Harry's bed knocking it over and causing Ron to hop up and down, howling with pain. Crookshank's fur suddenly stood on the end. A shrilll, tinny whistling was filling the room. The Pocket Sneakoscope had become dislodged from Uncle Vernon's old socks and was whirling and gleaming on the floor."
At first, we think that it was Crookshanks that made the Sneakoscope whistle. But later on in the story we find out that Peter Pettigrew (aka Scabbers) was causing it as he attempted to escape off of Ron's shoulder. (Thanks, Carmen!)
Goblet of Fire
After Harry sends the food package off to Sirius, he sees an eagle owl with a note in its mouth soar past Hagrid's cabin toward the castle. That eagle was carrying Voldemort's order to Crouch Jr. that he should stop Crouch Sr., who had escaped, at all costs.
"Should I go and get someone?" said Harry. "Madam Pomfrey?"
"No," said Dumbledore swiftly. "Stay here."
He raised his wand into the air and pointed it in the direction of Hagrid's cabin. Harry saw something silvery dart out of it and streak away through the trees like a ghostly bird. Then Dumbledore bent over Krum again, pointed his wand at him, and muttered, "Ennervate."
And then a couple of paragraphs down:
The sound of thunderous footfalls reached them, and Hagrid came panting into sight with Fang at his heels. He was carrying his crossbow.
Could this be the way members of the Order signal each other?
Edit: Indeed, it is the way the members of the Order signal each other!
Order of the Phoenix
The number Mr. Weasley punches into the telephone to get into the Ministry of Magic, 62442, spells "MAGIC" on the phone pad.
In Chapter 2, when Harry is trying to explain what has happened to Dudley he mentions Dementors and we find Aunt Petunia knows what they are:
"How many times do I have to tell you?" said Harry, temper and voice both rising. "It wasn't me! It was a couple of Dementors!"
"A couple of - what's this codswallop?"
"De - men - tors," said Harry slowly and clearly. "Two of them."
"And what the ruddy hell are Dementors?"
"They guard the wizard prison, Azkaban," said Aunt Petunia.
Two seconds of ringing silence followed these words before Aunt Petunia clapped her hand over her mouth as though she had let slip a disgusting swear word. Uncle Vernon was goggling at her. Harry's brain reeled. Mrs. Figg was one thing - but Aunt Petunia?
"How d'you know that?" he asked her, astonished.
Aunt Petunia looked quite appalled with herself. She glanced at Uncle Vernon in fearful apology, then lowered her hand slightly to reveal her horsy teeth.
"I heard - that awful boy - telling her about them - years ago," she said jerkily.
"If you mean my mum and dad, why don't you use their names?" said Harry loudly, but Aunt Petunia ignored him. She seemed horribly flustered."
That "horrible boy" was not James Potter, but as we learn in Deathly Hallows, Severus Snape. (Thanks, Meli!)
"Step over here," said the wizard in a bored voice. Harry walked closer to him and the wizard held up a long golden rod, thin and flexible as a car aerial, and passed it up and down Harry's front and back.
The thing that the guard uses to check Harry is a Secrecy Sensor. (Submitted by Kel.)
"Warrington of the Slytherin Quidditch team reported to the hospital wing with a horrible skin complaint that made him look as though he had been coated in cornflakes." (Chapter 30, Grawp, pg. 677)
Back in Chapter 6 ("The Noble and Most Ancient House of Black"), Sirius got "a bad bite from a silver snuffbox; within seconds, his bitten hand had developed an unpleasant crusty covering like a tough brown glove." He (Sirius) then says that it must have been Wartcap powder. Fred and George then snuck the snuffbox of Wartcap powder. They may have later given the powder to Lee and told him to put it on Warrington (or anyone he didn't like, really), as his symptoms seem to be very similar to Sirius'.
When Harry and company are visiting Mr. Weasley in St. Mungos, one of the portraits of old healers mis-diagnoses Ron's freckles as Spattergroit. This same disease was used by Ron, when he disguises the Ghoul as himself in Deathly Hallows. (Thanks to Vikki!)
In Chapter 28, "Flight Of The Prince," after Harry and Dumbledore's retrieval of the locket in the cave, it is discovered that the locket contains a piece of paper with a small note.
To the Dark Lord
I know I will be dead long before you read this
but I want you to know that it was I who discovered your secret.
I have stolen the real Horcrux and intend to destroy it as soon as I can.
I face death in the hope that when you meet your match,
you will be mortal once more.
Could the real locket be the same locket Harry found in Order of the Phoenix? In Chapter 6, "The Noble and Most Ancient House of Black," Harry finds a "a heavy locket that none of them could open..."
The initials of the person who wrote the note inside the locket are R.A.B. These initials can be Regulus Black, Sirius Black's brother, where coincidentally, a locket was found in Grimmauld Place.
In Chapter 2, while explaining to Snape that Voldermort trusts her Bellatrix says, "The Dark Lord has in the past entrusted me with his most precious..." She's talking about the Horcrux (Hufflepuff's Cup) stored in her Gringotts vault. (Thanks, Kenny!)
In Chapter 32, Ron, Harry, and Hermione are trying to get through the Whomping Willow, and Ron asks, "How - how're we going to get in? I can - see the place - if we just had - Crookshanks again -". Hermione retorts with "Crookshanks? Are you a wizard or what!?"
In Harry Potter and The Philosopher's Stone, Chapter 16; Harry, Ron, and Hermione are stuck in the Devil's Snare. Harry shouts at Hermione to light a fire (to make it release them), and Hermione responds with, "Yes - of course - but there's no wood!"" Ron then screams "Have you gone mad? Are you a witch or not?!" (Thanks to Erin!)
A witch or wizard's wand size is selected in direct proportion to their height. Hagrid is huge, and he has a 16-inch wand. Voldemort is tall, and he has a 13-inch wand. Ron is tall and he has a 14-inch wand. Harry is average height, and he has an 11-inch wand. Umbridge is extremely short, and her wand is described as being very short. (Submitted by Patil.)
credit - Mugglenet