Comment by Winkle on Harry Potter and the Methods of Rationality discussion thread, part 22, chapter 93 · 2013-07-07T18:30:21.485Z · LW · GW

The exception could conceivably be Blaise Zabini. From the TV tropes article titled "Ambiguous Gender" -

Harry Potter's Blaise Zabini got a single mention in book one, which sparked years of furious debate over his or her gender. Fan Fiction summaries often stated Boy!Blaise or Girl!Blaise, each with their own set of conventions and fanon personalities. (Boy!Blaise was often suave and Italian, Girl!Blaise often red-haired.)

It was not revealed until the sixth book that Blaise was male. While Eliezer chose to keep him male, he likely had the option to chose given the character's very uncertain past.