Friends: 16 Easter Eggs You Totally Missed
4. What Mario Isn't Telling You
The One Where Underdog Gets Away is classic early Friends, and one of the best Thanksgiving episodes that the show produced. A gag throughout the episode is that Joey, having done some modelling work for a health clinic, ends up on posters all over the city for the sexually transmitted disease VD, with the tagline 'What Mario Isn't Telling You'.
The gang joke about it, a girl won't date him because of it, his family don't allow him to go to Thanksgiving dinner because they think he has it, but it's contained within that episode. Except, a couple of episodes later, in The One With Mrs Bing, the posters are still around, as evidenced by the image above.