Season 9: Episode 196: The One with the Sharks
Ross points out that Phoebe's never been in a long term relationship, which freaks her out just as she leaves on a date with Mike. Ross tracks down Mike later to explain why Phoebe was so disturbed, but just makes things worse with a fictional story about a six-year relationship she had. Chandler has to stay the weekend in Tulsa, so Monica flies there to surprise him. He's watching porn when she arrives, but he changes the channel quickly, which makes her think he gets off to shark documentaries. Joey meets a hot woman (Hayley) and starts dating her, but he recognizes her apartment and thinks he's slept with her before. She obviously doesn't remember it so he finally confronts her about it, only to find out he previously dated her roommate.