Other mistake: In the scene where Rachel is going to take out the trash, Monica tells her the garbage room is the 3rd door on the left. However, when Rachel comes back with the pizza box and Monica leaves for the garbage room, she leaves the apartment and walks towards the right - there are no doors that way, just the stairs.
Continuity mistake: When Jill drops her shopping bags on the coffee shop table, the small blue Tiffany's bag is visible on the end of the 'line'. But it is not visible in any shots after Rachel comes in. Additionally, when Rachel grabs the bags and leaves, Ross seems to pick the little blue bag up off the floor, though it is not shown how the bag got there.
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.