Revealing mistake: When Rachel holds up the Polaroid she took of her assistant, it appears to have been one of the actual photos taken in the earlier scene; you can see the edge of the set behind him, and the overhead lights in the black space to the right of the photo.
Visible crew/equipment: After Monica tells Chandler the secret about Nancy from Phoebe's old massage place getting fired, when it cuts to the wideshot of the apartment while Rachel's standing by the fridge, we can see the camera at the left side of the screen.
Add timeSuper Grover
Continuity mistake: Ross has always said that Carol was his first (and only, until Julie) sexual partner, but in the episode where they tell each others secrets Chandler says Ross slept with the cleaner when they were at college. Some people say that Ross had kept it a secret, and that's why it had never come out before, but considering that he categorically states several times over series one and two how he'd only EVER been with Carol, getting quite upset at times, that explanation just doesn't ring true.