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.
Continuity mistake: When Chandler answers the phone and it's Gandalf calling to cancel, Ross is just opening the fridge, but the shot cuts to Chandler with the phone and Ross is putting two beers down on the side next to him - too quick to have got them there from the fridge.
Continuity mistake: In the scene where Rachel and Ross are playing poker head to head, Rachel borrows $7 from Monica's purse and then puts the purse down on the table. She puts it down on her left hand side but it then switches to her right hand side without her touching it and then back again.
Plot hole: David Schwimmer is 6'2" tall and Jennifer Aniston is 5'6". There is no way that clothes that fit Rachel would also fit Ross. However when he puts on the clothes that Rachel bought it's obviously not Rachel's size. If it really were, it would be a lot tighter and shorter (Ross could probably wear it belly free).
Continuity mistake: In the scene after Monica pretends to drop socks in the hallway in order to prevent Rachel from telling Ross she still loves him, Rachel goes back into the apartment and locks Monica out. Monica then starts to pound on the door with her bare left hand. The scene changes to the apartment and Ross opens the door to let Monica in. All of a sudden Monica is pounding the door with her right hand wrapped in a sock.
Continuity mistake: When Joey is reading his speech about all the giving and receiving, Chandler has his legs on the table at the coffee shop. You can see his shoes on the table when they show Joey, but when they change the shot to Chandler his legs are on the floor and crossed.