Lily: How did it happen?
Ted: Oh, it was the stupidest thing. I mean, we were just hanging out, you know, being funny...
[flashback to Ted and Karen having a pillow fight in bed:]
Ted: You're the fan of 2002 Chiantis.
Karen: Ew, no, you're the fan of 2002 Chiantis. Wait. What is this?
Ted: Looks like one of Robin's earrings. Huh, that's weird. You're the fan...
Karen: How did it get in your bed?
Ted: I don't know.
Karen: Have you been sleeping with Robin?
Ted: What? No, of course not! It, it probably got mixed in with the laundry or something.
Karen: Why should I believe you?
Ted: Um, maybe because I've always been faithful to you while you've cheated on me, like, six times, bitch.
[present:]
Lily: Oh, snap!
Marshall: Cold-blooded!
Ted: Right? Right? I totally should have said that.