Harry Potter and the Half-Blood Prince

Fantastical Facts

We here at the Lexicon are working overtime to add all the new facts into the pages of the site. In the meantime, this page will serve as a place for us to post some of the more intriguing new bits of information we and many of you have noticed.

new Minister of Magic - Rufus Scrimgeour, almost certainly the Scrimgeour mentioned in OP as asking pointed questions of Tonks and Kingsley about Sirius Black. According to HBP, he was head of the Aurors, so it makes sense.
We get to see how the Prime Minister is notified of things. Since this book takes place in 1996, the Prime Minister would be John Major. The President of a far off country might or might not be the U.S. president. Incidentally, the US President at that time was Bill Clinton.
spelling error in chapter 3? "The misty fug his breath had left on the window..." Should that be 'fog' or not? The word 'fug' is actually an informal British term meaning an airless, smoky, smelly atmosphere. Doesn't seem to fit, really.
Definite spelling error in chapter 1, on page 10, end of the 8th line: the word should be 'sight' but it's spelled 'site.'
No, the Rupert thing isn't a spelling error, it's supposed to be a bit of a joke, as if Slughorn can't remember Ron's name.
Romances galore:

Ron and Lavender Brown
Ron and Hermione
Ginny and Dean Thomas
Ginny and Harry
Hagrid and Olympe
Lupin and Tonks
Bill and Fleur (cute line: "I am good-looking enough for both of us, I theenk!")
Filch and Pince (now there's a pleasant pair)

Deaths galore:

Aragog
Amelia Bones
Emmeline Vance
Hannah Abbott's mom
Florean Fortescue
Gibbon the Death Eater
and then there's that other one

Harry inherits everything Black owned, including number twelve Grimmauld Place--and Kreacher
Neville's grandmother's name is Augusta Longbottom, and she brags about him in the paper!
The whole Gaunt family...look what Slytherin's family line came to...
Horcruxes are key, and Voldemort apparently made six of them (not counting the bit left inside of him)

special note about the locket - traced to a hidden cavern, guarded at the center of an underground lake by Inferi, but it was actually a dummy placed by a mysterious "R.A.B." - SVA's guess is that Harry will have to shake down Mundungus for that one. (MLW's guess is that Kreacher should not be forgotten.)

The Marauders are referred to specifically by that name by Harry. So it's official now and I can finally call them that in the Lexicon! Yay!
Voldemort was born on New Year's Eve in a Muggle orphanage, although it's not absolutely clear if the birth happened before midnight...still ambiguous, that one.
Contradictory text: Dumbledore becomes Headmaster of Hogwarts ten years after the murder of the Riddles, which would be approximately 1955. That tallies with McGonagall becoming the Transfiguration teacher in 1956, as we are told in OP. However, this passage in PA18 is now an error, since it makes no sense:

new teacher: Horace Slughorn, who teaches Potions when Snape takes over Defense Against the Dark Arts, is a 'good' Slytherin. Devious and selfish, sure, but not evil. He was head of Slytherin house in his day and takes that position when Snape leaves.
Contradictory text again: In Goblet, Harry runs up the marble staircase toward the second floor to go to Dumbledore's office. If he were just going to the top of the stairs, it would be to the first floor, since that's what's at the top of the stairs (the entrance hall and great hall are on the ground floor, to clarify for US readers). In this book, Dumbledore's office entrance is clearly on the seventh floor.
The DADA position was in fact jinxed by Voldemort when he was turned down for the job. No teacher has stayed more than one year for 25 years, we are told, although Quirrell clearly was the DADA teacher for at least two years, more likely three. Another continuity error, I'm afraid.

UPDATE: Two possible solutions to this error have been suggested by readers:

Quirrell taught a different subject before his leave of absence
Quirrell was DADA teacher for more than one year, but not more than one CONSECUTIVE years.

Neither of these solutions is perfect, but they could be worked around to provide an "out" for Jo if she needs one...

Wormtail is staying with Snape in a house on a street called Spinner's End in a derelict Muggle town. Is that Snape's home, then? Where he lives during the summers? What a depressing place...kind of fits with the life that Tobias and Eileen were living, though.
Dumbledore's favorite flavor of jam is raspberry
New drinks (and there's a lot of drinking in this book!)

elf-made wine, bloodred in color
Madam Rosmerta's oak-matured mead

Arthur calls Molly "Mollywobbles" when they're alone together (laughed out loud when I read that)
Inferi - animated corpses - the word is Latin for "the dead" (the plural form of the Latin word 'inferus'). One typically wouldn't refer to the dead in singular and the word inferus isn't used to mean a dead body (which would be 'cadaver' in Latin). So Jo invented a singular form, Inferius, to refer to one animated corpse. We Muggles might refer to them from our folklore as zombies or the undead.
the editor of the Daily Prophet is Barnabas Cuffe
Dumbledore visits the bathroom and stays in there awhile, reading the Muggle magazines. "I do love knitting patterns," he says.
What's with all the spiders? Snape lives on a street called Spinner's End (chapter two). Aragog dies (a true spinner's end, in a sense, in chapter twenty-two). Harry imagines Slughorn as a fat spider. And Dumbledore has spiders all over his hat in the broomshed when he and Harry are discussing Sirius and Harry's reactions to events).
Arthur and Molly got married during the first war (which would be during the 1970s). It would seem from the way it's mentioned that they eloped in rather a hurry. Just to clarify, Bill was born on November 29, 1970. Exactly how these dates all fit together is uncertain, but I'm figuring it means that they eloped in 1968 or so, while Voldemort was coming to power.
Ginny's nickname for Fleur, "Phlegm," is going to be a fun one for the translators.
Madam Pince and Filch were standing together at the funeral. Now isn't that an interesting match...
Grawp is, well, tame. Hagrid was busy!
U-NO-POO. I'm with Ron...brillant. And same goes for everything in Weasley's Wizard Wheezes.
Who is R.A.B.? My guess: Regulus Black. That seems to be everyone else's guess, too. We're all so clever, aren't we?
So why did Dumbledore trust Snape? A bit of speculation -

Did Snape take an Unbreakable Vow saying he'd never betray Dumbledore? If so, who was the Bonder? Aberforth, maybe?
And Snape, after more or less being tricked into vowing to kill Dumbledore, went to the Headmaster and explained the predicament he was in. One of the two of them had to die, and Dumbledore quickly realized it would make more sense for it to be him than Snape. So they planned for Dumbledore to spend the year getting Harry up to speed while Snape kept Draco under control, then Snape killed Dumbledore. At the last moment, Snape almost couldn't do it and Dumbledore had to tell him "Severus, please..." Now Snape has proven beyond a shadow of a doubt that he is Voldemort's best and most wonderful supporter, and is therefore in the best possible position to help the side of good.
Or maybe Snape is just really, really evil.