Continuity mistake: S3E1; Ross is telling Chandler about how woman tell each other everything in Central perk. Chandler puts a book about trout on the table and then talks to Ross, then the camera zooms out and the book has moved more to his left and a coffee mug is in front of it.
Continuity mistake: At the beginning, Alice and Frank tell Phoebe that they have been trying to get pregnant. When Alice says, "You know, no one's getting any younger", Frank looks at Alice and laughs. In the next shot however, he is looking at Phoebe and doesn't seem to laugh.