Trivia: In The One With The Superbowl Part 2 the gang are down at the movie set of Marcel's film. Joey asks Phoebe to walk with him so he can show off his acting skills to the director. If you look closely you can see that the 'director' and the people surrounding him are the actual producers of Friends - Kevin S. Bright, Marta Kauffman and David Crane.
Trivia: Phoebe loses her job as a singer at Central Perk. As a protest she sits outside singing for money. Rachel comes out to give Phoebe a cup of coffee. They talk for a while and Phoebe reveals that one of her donations for playing "Smelly cat" was a condom. A moment later a guy comes running to get back the condom, played by Giovanni Ribisi. He's not meant to be her brother, as her brother says when she meets him that he's never been to the city before - just a random incidence of an actor playing different roles across series.
Trivia: Rachel says about her mother who thinks about a divorce and wants to be like her daughter : "Well, couldn't she have just copied my haircut?" A little reference to the fact that the Rachel haircut was one of the most copied one at the time.