The character of Joey was of normal intelligence for the early seasons before the actor decided to play him more "dim"
The crew often switched up the drawing on the Magna Doodle, so the design can often be seen changing between shots
In "The One Where Mr. Heckles Dies," Chandler calls Joey "Matt" (the actor's real name)
The wedding dress Monica picked up for Emily isn't actually the one she wears at the wedding
Monica's apartment changes number from 5 to 20
The reason the gang had no trouble sitting at the same spot everyday at Central Perk was that it was reserved for them
Pheobe claims to not understand Italian when speaking with Paolo, yet speaks fluent Italian with Joey's grandmother in a later season
Ross is said to be 29 for over 3 years on the show
Ross also has two birthdays: First, he claims it’s in December, then says he was born on October 18th
Rachel's last name is “Green” in the credits, but the sign on Rachel’s door in Bloomingdale’s says ”Rachel Greene”
There's a potato in the cake stand and there's not much else we can say about that
Ever noticed that Central Perk had employee uniforms?
Joey's food stains tend to change and diminish between shots