Back in the heyday of Harry Potter fandom, when we lived in eager (desperate) anticipation of the next book, Rowling gave quite a few interviews. It was clear from the answers she gave that she had the details of her created world and her plot lines very carefully planned out. However, inevitably some of the answers she gave turned out to be incorrect, either because she dropped a story line or changed her mind.
Just for fun, here are three Rowling interview quotes which gave information she later changed or left hanging:
Q: Has Harry ever used the Internet?
JKR: No. He’s not allowed near Dudley’s computer and Dudley’s the only one who’s got a computer. He gets beaten up if he goes too near the keyboard. So no, he’s never used the Internet. I use it a lot but not Harry. Wizards don’t really need to use the Internet but that’s something that you’ll find out later on in the series. They have a means of finding out what goes on in the outside world that I think is more fun than the Internet. Could anything be more fun than the Internet? Yes!
So what was this “more fun than the Internet” thing. We never did find out “later on in the series.”
ES: [I]s he dead?
JKR: Yeah, he is.
ES: … [C]an you give us some backstory on him?
JKR: I’m going to tell you as much as I told someone earlier who asked me. … He said, “Is it coincidence that he died in 1945,” and I said no. It amuses me to make allusions to things that were happening in the Muggle world, so my feeling would be that while there’s a global Muggle war going on, there’s also a global wizarding war going on.
This is a great example of Rowling changing her story as it is written. Clearly at this point in 2005 she intended for Grindewald to have died in the duel with Dumbledore in 1945. During the writing of book seven, she changed the story to have Grindelwald still alive in 1998 and able to speak with Voldemort about the elder wand.
Q: Will there be, or have there been, any “late blooming” students in the school who come into their magic potential as adults, rather than as children?
JKR: No, is the answer. In my books, magic almost always shows itself in a person before age 11; however, there is a character who does manage in desperate circumstances to do magic quite late in life, but that is very rare in the world I am writing about.
Fans came up with all sorts of interesting theories about who this might be. The top contenders were Dudley and Petunia. However, Rowling changed her mind and never did include this in her story.
Bonus quote (just because it makes me laugh):
Q: What do you most like about Quidditch?
JKR: That would probably be the violence.