Nikki Heat

Castle tells Beckett that he didn't sleep with Natalie because it was "Too meta." However, because he breaks up with Gina in the next episode, we know that he was still in a relationship with Gina at this time, and that's why he wouldn't have slept with Natalie. This was aired out of sequence. "Poof You're Dead" was written to air ahead of it. From the writer's view, Castle had already broken up with Gina when Natalie tried to seduce him. (Wikipedia, List_of_Castle_episodes).

Actually, the issue with Castle is that Natalie is portraying a character from one of his books and modeled after Beckett. In effect he would be sleeping with his fictional creation (which for the writer is another dimension), and by extension his real life partner Beckett.

References Hogan's Heroes (1965)