Continuity mistake: Towards the end, during a scene at Ross and Rachel's, Sandy picks up two puppets and puts them on each hand. He starts pulling the blue one onto his left hand, but when the angle changes there's a yellow one on his left hand, and a blue one on his right. It cuts to Ross, then when we go back to Sandy they've changed hands again. (00:24:30)
Continuity mistake: When Ross walks in on Sandy and Joey playing the recorders, Joey puts his recorder down on the table in front of him, when Ross storms out of the room, the recorder is in a totally different position and also on the far end of the table. (00:24:55)
Continuity mistake: The hallway outside Ross' new apartment (series 6 onwards) always changes. Sometimes, the apartment is at the end of a dead-end corridor, and there is a plant outside, covering up the wall; sometimes you can go past the apartment and turn left or right; or, as in this episode, you can only turn right immediately after you have passed the apartment. (00:26:00)
Continuity mistake: When Phoebe is trying to explain to David that she really cares about Mike the door behind David is closed, but when Mike walks in that door is wide open, and it remains open until it's suddenly closed when Mike warns David that if he ever goes to Minsk he'd better watch out.