Public Service Announcement (this has no relation to the prompt): When Hemingway (I think, but it doesn't really matter) said, "Write what you know," it was a critique of Elizabeth Barrett Browning, who had said, "Write what you don't know." In other words, it would be like me saying, "You are therefore you think." It may or may not be true, but it was a critique of an idea that had been set in stone and codified. Codifying that idea, in turn, defeats the purpose.

To be more succinct, When I hear, "Write what you know," I reach for my gun.