what episode is thirteen wearing that hat???

I know exactly which hat you are talking about, because it is soooo awesome! She has worn it in at least 3 episodes over the past 2 seasons. (Its sad that I know this, but I kind of have a photographic memory). She first wore it in episode 10 of season 4: "Its a Wonderful Lie" (You see her in at as she enters the hospital and gets on an elevator while talking to Kutner); the hat is next seen in season 5, episode 12: "Painless" (She is wearing it when they are doing a differential in Cuddy's baby's room); and it was last seen in season 5, episode 15: "Unfaithful" (At the end, when she enters her apartment and makes out with Foreman). Hope this helps. :)