Series 3, Episode 8
Joey tells Chandler how he saw Janice kissing her husband (at the conclusion of the previous episode) and he puts her on the spot to find out what her intentions are. Phoebe has a trip to the dentist but is afraid someone will die as a consequence, and Monica accidentally bumps baby Ben's head and tries to cover up the evidence with a silly hat.
The scenes between Chandler and Janice do appear to be quite serious and actually made me feel emotional. Please don't tell anybody!