Season 2: Episode 22 - Something Blue

Something Blue: At Marshall and Lily's wedding reception, Barney discovers Ted and Robin have a secret that they've been hiding for two weeks. Neither of them wanted to draw attention from Marshall and Lily's day so they decided not to say anything but after enough prying, they agree to tell Barney. While Barney speculates Ted and Robin are married or pregnant, the real secret comes as a big surprise.  Meanwhile, Lily gets drunk at the reception because she and Marshall were too busy to eat and gives in to all the clichés (the conga line, the bouquet toss, etc.) that she wanted to avoid.