Nate brings a drunk Serena to get cleaned up in a flashback of the previous year. They get into a water fight with the shower head. Blair checks on them... and gets hosed down herself!
Serena: What's Chuck doing today? Blair: Why are you asking me? Serena: Well, you guys are friends. And, Blair, you know you can tell me anything. I'd be the last person to judge anyone. Blair: With good reason. Serena: Blair, I saw you with Chuck. Blair: [pauses] I don't know what I was thinking. I mean, sleeping with him once, maybe I could understand. But twice? Serena: Wait, you slept with him!? Blair: Shhh. Serena: Blair!