Grey's Anatomy Season 3 Episode 6

Nancy: So, tell me about the slutty girl. (Derek gives her a look) Fine, the slutty intern.
Derek: It was the 'slutty' part I had a problem with.
Nancy: Kathleen says she's not even single.
Derek: She is single, she is smart and she's a lot of things. And she's none of your business.
Nancy: Wow. I've never seen you like this over a girl before. Not even Addison.
Derek: I've never been like this over a girl before, especially Addison.
Nancy: Oh, don't be bitter. I mean shame on Addison and all that. I mean, he's Mark, Derek, what'd you expect? He's just Mark. And who hasn't gone there once or twice, right?
Derek: What'd you just say?
Nancy: Everybody sleeps with Mark, it's like a right of passage. Derek I get it. I get that they made a terrible mistake.
Derek: It wasn't a mistake, Nance, it wasn't a mistake, okay? It was wasn't one night. It was months. Did you know about that?
Nancy: No. I had no idea.
Derek: Then shut up about it and eat your lunch.