An insult aimed at J.K. Rowling by wannabe literature snobs who like to spend all their time critiquing teen fantasy books because they're bitter that nobody wants to publish their own rubbish. The root of the insult is that J.K. Rowling uses a lot of the plot device deus ex machina (in this case, going back and changing the ending as in reversing time to fix a problem or adding/changing a detail hence "just kidding!"), widely considered a literary cop-out but generally accepted in literature intended for children by all but the most persnickety critics.
Hermione goes back in time in Harry Potter book 3 so that Sirius Black can escape prison.
Nickname used to make fun of J.K. Rowling and annoy Pottertards who defend Rowling's failings as a writer using the "It's a just children's book!" excuse, even though Rowling herself stated that the last books in the series were aimed at older readers.
What was thinking Just Kidding Rowling when she wrote that awful epilogue!?