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: 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.
Continuity mistake: In the episode where Rachel goes to tell Ross she loves him (in England) she leaves her room with a brown sleeveless vest and a cardigan over her shoulders. The cardigan then disappears in all other shots until after Phoebe says "he's in love with the British chippy". Her leather jacket also goes from being in her hands to over her arm.
Continuity mistake: In the previous episode, Monica's Christmas tree is up in her apartment. After the party, Joey kisses Janine. In the next episode, he walks into Monica's apartment and says he just kissed her. However, Monica's tree is down and they have all changed their clothes. Surely if he just kissed her then it is still the same night of the party, therefore the tree should be up and Monica, Ross and Joey should be wearing the same clothes from the party.
Continuity mistake: Near the start when Janice is touching Joey's cheek in his apartment, Joey is seen from the front, and he is not holding anything in his hands. He is then shown from the side and he is holding a box of cookies or something similar.
Factual error: When the gang all see the beach house for the first time, it's covered in a layer of sand, which is supposedly left over from a recent flood. Well, if there really had been a flood, there would have been mold on the walls and debris or junk from the outside, and the sand wouldn't have been nice and clean and dry; it would have been moldy. And there would've been a pattern, likely on the walls, of where the water rose up to and where it receded. Not only that, but how did it get in? All of the doors and windows are intact.