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.
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.