Season 4: Episode 80: The One Where Chandler Crosses The Line
When Chandler accidentally catches a glimpse of Kathy getting out of the shower, his frustrations reach their limit. When Joey's late for a date with Kathy because he's on another date with someone else, Chandler makes his move - the two kiss. Riddled with guilt, Chandler re-furnishes the apartment. When Kathy breaks up with Joey, he suspects there's another guy. Chandler admits that he's the other guy, which creates tension between the two friends. Meanwhile , Ross rediscovers his musical "sound," but he's actually not as talented as he and Phoebe seem to believe. Phoebe doesn't want to perform became she feels dwarfed but Ross' musical gift.