The primary school kids thought they were waiting for a member of staff to give them a book reading, and were told to count down the last 20 seconds before midnight.
And then, as if by magic, the Harry Potter author appeared too!
'I'll never wash again!'
There was stunned silence as JK walked in.
Then people realised who she was, and there were a lot of gasps and cheers.
An excited fan dressed as Harry
JK had with her a huge wooden chest full of copies of Order of the Phoenix.
The children excitedly lined up to be given a copy by JK, who also signed their books!
Afterwards, they held up their copies for the adults to see.
Most of the children said they were going to start reading straight away!
JK told Newsround's Lizo: "I think it's miraculous given the number of books that have been produced, and the number of people who are involved that everything hasn't been revealed. So I'm really happy."
She turned up at the bookstore in Edinburgh because it's her home town and it's where she wrote all of the boy wizard books.
All of the children were dressed up as wizards, witches or their fave characters, and were very excited.
Ben, 10, who was dressed as Fang, said: "I didn't expect this at all. It's amazing to meet such a famous author. It's a once in a lifetime experience."
Flora didn't think it was JK for a while
Flora, 10, said: "I didn't believe it was her - I thought she was one of those doubles!"
And Sidra, nine, added: "I shook her hand - I'm never going to wash it again."
Global rush
Across the world, milions of hyped-up Harry fans queued at book shops, desperate to get their hands on the first copies of Order of the Phoenix.
Children (and adults!) queued for hours to buy their precious copies of book five, with many pledging to stay up all night to read it.
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Webcam: see fans in the Potter queue
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Review: Order of the Phoenix
