Plot hole: When Monica falls off the chair you can see that she is wearing a white bra because you can see some of the cup and the strap (when she says "you're not getting it". Towards the end of the episode (still the same day) we find out that Monica is meant to have the same bra on as their new cleaner, Brenda. The bra she is meant to have on is 'pink with the lacy cup'. Rachel notices that Monica is wearing that bra and she shows Monica. What happened to the white bra she had on a few scenes before?
Plot hole: The times in this episode vary a lot. Monica and Phoebe first bet about Rachel giving birth, then it goes to the scene with Joey and Chandler at the movies. It then returns to Monica and Phoebe the next day with Monica losing the bet. However, a little while later, it goes back a day and shows Joey and Chandler at the movie premiere again.
Continuity mistake: In the scene with Robin Williams and Billy Crystal, there is a shot of Rachel, and her hair is hanging straight down. The next shot of Rachel is from a different angle and there is a section of hair behind her ear; in the shot after that it is hanging straight again, and is then behind her ears again.
Continuity mistake: There is a scene in Central Perk where Rachel confronts Chandler about all the kissing he's been doing. She raises her hands up in front of her and tells Chandler to hold on to stop him from getting up from the couch. As he goes to sit back down on the couch she lowers her hands to her lap. The shot cuts to an angle behind Rachel's chair and she is lowering her hands again.
Continuity mistake: When Ross and Rachel are arguing about Rachel telling Phoebe about his fantasy, Ross takes a popsicle out of the freezer and bites it. In the next shot, the popsicle is whole again, with no bites, and Ross takes another 'first' bite out of it.