The Pottermore conundrum for fanfic writers.
Now that details are being revealed from the new Pottermore site and the new info that JKR is making available there, all kinds of previously held canon facts are being thrown into question. For example, Minerva McGonagall was always estimated as having been born in 1924 or 1925, and that she began teaching at Hogwarts in her early thirties. (This was based on a statement Rowling made in an early Scholastic.com interview, in which she said that McGonagall was a “sprightly 70,” presumably in the then-current book, Goblet of Fire, which took place in 1994-95. Subtract 70 from 1994 and you get 1924, and since her birthday is in October, Minerva would have been 70 for most of the 1994-95 school year if she were born in October 1924.)