Continuity mistake: When Janine goes into the hallway to apologize to Monica about her behavior at dinner, she runs into Monica who says, "What did I tell you about the hall?" As she says this, she is pointing her right forefinger at Janine and her right shoulder is leaned in closer to her. When Janine says "I was just coming over here to apologize for my behavior," the camera cuts to Monica, who is now standing with her left shoulder leaning in toward her. The camera cuts back to Janine as she says "I'd really like it if we could be friends" and then back to Monica for her next line, where she again has switched to her right shoulder leaning toward Janine.
Factual error: This mistake is from series 6 onwards, with Ross' new apartment: There is an impossibility with Ross' kitchen, but not all of the time. It depends on the layout of the hallway outside his apartment. If the hallway continues along for a bit before going left or right, then the kitchen should stick out. However, when the hallway is a dead end, the kitchen fits ok.
Continuity mistake: Toward the end of the episode, Rachel is back in Ross' apartment, trying to mend the ornament she broke when she was in there earlier. Ross comes in unexpectedly and she tosses the broken pieces into the lightshade, leaving the glue on the side table. In subsequent shots, the glue goes from lying on its side, to standing up, a number of times.
Continuity mistake: When Rachel and Joey are at Joey's apartment throwing wet paper towels at the entertainment unit, Phoebe comes in to ask Rachel to switch back to Monica's apartment. During the scene, Phoebe is standing in front of the entertainment unit, and the wet paper towels' number and positions change between shots.
Continuity mistake: When Chandler and Monica are talking about Chandler's shark obsession, his arms keep changing with each shot. When the shot is from behind (facing Monica) they are on the couch, and when the shot is in front (facing Chandler), his arms are folded in front of him.