Continuity mistake: When Joey says "we'll call that plan B, all right?", Monica has her chin in her hand. When it cuts to a different angle, she's got both arms on the table. Rachel also goes from having both hands clasped under her chin to having one arm resting on the table and one reaching for coffee.
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.