Mine was "He's a she?"

I knew Harper Lee was alive from listening to the excellent audiobook On Writing, authored and narrated by Stephen King. He stated his astonishment that To Kill a Mockingbird was the only novel she ever wrote, but in a manner that never conveyed her gender (that I recall, anyway).

I even read the book years before without realizing that the author was a woman. It's the only assigned reading I have ever had that I couldn't put down.