This is really across seasons 3-5 as far as I know: Ross remains the same age (29) for all three years. In season three he tells Chandler "No thanks, I'm 29" when offered chocolate milk, in season four when the guys try to party without Gandalf they all say they're 29, and in season five when he's trying to reconcile with Emily he says either he does or he would get divorced for the second time before age 30
The show is set in New York. Joey was a cast member on Days of Our Lives, which is filmed in Los Angeles, but is still always around
The Chick changes breeds and gender a couple of times. It starts out as a full size White Leghorn. During the adolescent phase it is a female White Silkie. This is a bantam or small size chicken. In maturity it is a male White Leghorn.
The number of the flats near the start of the first series was 4 and 5. After a few shows they changed to 19 and 20. [The producers realised that the numbers 4 and 5 were for apartments on the lower floors and had shown Monica's apartment to be much higher up so they changed the numbers - it's still a mistake but there's the reason.]
In the pilot episode, the guy who Rachel left at the altar is called Barry Finkel. In all the other episodes his name is Barry Farber
In the scene where the friends scoot the table over to see Monica's date with "Paul the wine guy", the cup in front of Chandler changes position
In the pilot episode we see two completely different backgrounds in the two windows in Monica's apartment than we do in the rest of the series
When the camera moves in to look through the window of Central Perk we see several customers, there is even a non-Friend sitting at the table to the left of the Friends' sofa (as we see it). When the shot changes to inside, nobody is there but them
When Janice comes into the coffee shop to talk to Chandler, she brings several shopping bags which she sets by the couch. But when she gets back from the restroom and Phoebe breaks up with her for Chandler, she leaves the coffee shop without getting her bags from in front of the couch.
In the last scene of the episode when the friends are sitting on the sofa in Central Perk, Ross has the ice on his forehead because he hit it on the machine in the launderette. At first Rachel is holding it on Ross' head, then in another shot Ross is holding it, then Rachel again, and then in the final shot Ross is holding it. It may be that they did keep switching but it's unlikely and looks more like the scene was filmed twice and the mistake was made when it was edit