Season 6, episode 9: "The One Where Ross Got High"
When Monica reveals that she hasn't told her parents she and Chandler are living together, he tries to win them over when they come over for Thanksgiving dinner. We later learn it's because seemingly perfect Ross blamed Chandler when his parents caught him smoking pot in college, but what truly makes this episode hysterical is when Rachel is given dessert-making responsibilities and inadvertently makes a English trifle with berries and beef after not realising her cookbook's pages are stuck together. Everyone's first-taste reactions and attempts to pretend they like it are beyond hilarious.