Joey does some of his Days of Our Lives scenes with an actor named Paul Logan, who has been on the show in real life.
When Rachel meets Gavin in her office, on the nameplate on the door her last name is spelled "Greene". In most of the other episodes (except "The One With The Invitation" and "The One With Phoebe's Rats"), her last name is spelled "Green".
Joey says that they 'use the same extras for the different parts.' This is actually the same case in Friends, for example, Joey's Potential Roommate also played the person who he flirted with on the set of Mac and Cheese.
Funny how Joey says the extras can be used once and again and during the credits at the end the security guard that chases Phoebe in the studio is the same that kicked her out the casino in Las Vegas.
Ross asks to borrow a blue tie from Chandler because he can't find his. Rachel can be seen wearing it in her next scene.