Continuity mistake: When Rachel finds out that Ross and Julie are getting a cat, watch Rachel's neck. When she says "together?" she's not wearing a necklace; cut to Ross and Julie for a fraction of a second, then when it cuts back to Rachel saying "both of you?" she's suddenly wearing a necklace. It then disappears again when she says "isn't that just lovely?" hanging off the side of the tray she's holding.
Visible crew/equipment: During the scene when Phoebe breaks into the other Phoebe's house and gets caught, the older Phoebe was telling the younger one that the man in the photo is not Frank Buffay, but is Chuck Mangioni. Right after that, as Phoebe 2 walks towards the camera, you can see that the door behind her moves open, and there is a visible shadow of someone who walks by that doorway.
Revealing mistake: In the scene in the coffee house, Phoebe is talking to Monica who is drinking a cup of coffee. Towards the end of the scene, the camera is on Phoebe who is smiling & laughing at Monica. In the right hand corner is a side view of 'Monica' however, it is not Courtney Cox in the scene, but a completely different actress. [Widescreen version only, although the "wrong" nose is visible in the original].
Joey: Do you know something?
Rachel: Do you know something?
Joey: I might know something.
Rachel: I might know something too.
Joey: What's the thing you know?
Rachel: Oh no, I can't tell you until you tell me what you know.
Joey: I can't tell you what I know.
Rachel: Well then I can't tell you what I know.
Joey: Okay, fine. [Pause.] You don't know!
Rachel: All right, how about I go over there and I will walk into Chandler's bedroom and I will see that thing that I think that I know is actually the thing that I think that I know!
Joey: [gasps.] YOU KNOW!
Question: When Ross and Rachel are fighting, Chandler hides behind the door and bursts out saying "I knew it." When Rachel says, "It's not that common, it doesn't happen to every guy and it is a big deal.", is this just a Chandler moment or is there another joke I have missed?