Continuity mistake: Chandler comes down to find Monica and sees that she's playing some game and having fun. He makes a sad and dejected face. The scene cuts back to Monica, and then to the exact same shot again of Chandler making the sad and dejected face.
Continuity mistake: After Chandler shouts at everyone because they are complaining about his smoking, there is an awkward silence and we see a shot of Rachel leaning on the sofa with her arms spread. In the next shot she isn't leaning on the sofa anymore.
Continuity mistake: When Monica is in Richard's room she lights candles then realises Richard's home with another woman so blows out the candles but misses one. Next she hides under the duvet waiting for Richard. When he comes in all candles are unlit but she is still under the bed.
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: Phoebe knocks over a vase and has to clean it up because "Let's face it; we're at Monica's." When she turns back around and notices one of the babies is missing, the baby on the right (in the striped top) falls over towards the other baby. A split second later when the shot changes the baby is sitting upright again.
Continuity mistake: In the deleted scene on the DVDs Chandler is leaving the security interview with his holdall over his shoulder by the short strap. The camera cuts to outside the room and the holdall is now level with his legs. (Mistakes in Deleted scenes are valid if they are only in that scene which this is.)
Continuity mistake: When Joey and Phoebe are talking about his co-star spitting on him, and Phoebe says that she gets spit on at work, Joey puts his hand on her arm. Then in the next shot, when Monica and Chandler are walking along, Joey puts his hand on Phoebe again.