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.