That a few of the names from the books are actually names of constellations? For example, Sirius is the name for the Dog Star (Canis Major) and Lupus (or Lupin, in the book) is the constellation of a wolf.
That the original release date for the Harry Potter and the Half-Blood Prince film (21 November 2008) was pushed back to July 2009? Instead, the movie Twilight was put in its original release date.
That Hermione Granger's original last name was 'Puckle', but was changed by Rowling before the book was released?
That Draco is Latin for dragon, and the first part of his last name, "mal," means "evil," so if "Draco Mal" was translated to English it would mean "evil dragon?"
That Dumbledore saw himself with his sister, Ariana Dumbledore, (not a pair of socks like it says in Book 1) when he saw himself in the Mirror Of Erised. He always feels guilty about it because he feels that he is responsible for her death.
That George Weasley will never be able to produce a Patronus ever again because of Fred's death?
