The customer at the coffee bar who overhears Joey say, "Sorry, this seat's saved" is J.D. Lewis. He is one of the top acting coaches in the world.
Although there's no evidence as to who really created the joke, Ross has tried to claim certain ideas as his own several times throughout the series. One notable instance is when he tries to convince his students that Jurassic Park was his idea.
Joey tells Gunther he doesn't want to go from being a regular on a soap opera to a waiter. In The One Where Eddie Won't Go (1996), Gunther tells Joey he used to be Bryce on All My Children (1970); his character was killed off by being buried in an avalanche.
The movie that Joey is watching when Monica and Chandler find him in their apartment is Ray Harryhausen's The Beast from 20,000 Fathoms (1953).
In the second half of the Central Perk scene when everyone finds out that Joey works there, Phoebe is one of the three customers behind Joey at the counter.