Continuity mistake: When Phoebe tells Ross that she wants to hear his "sound", she has her guitar in her lap. The shot changes, she says "Play that funky music white boy," and her guitar is missing. In the next shot, Ross runs out and her guitar is back on her lap again.
Continuity mistake: When Rachel is telling Monica and Phoebe about her night out with Joshua the previous evening, Monica is holding a glass of orange juice. To begin with the glass is half full, but then changes to being almost full to the top, and then back to half full.
Continuity mistake: In the episode with Chandler in a box, when Rachel is showing Ross all the things she had kept from their relationship in a shoe box, her hair is tucked behind her ear in most shots, but when she says "I keep the gifts that matter", it's all loose, then goes back to how it was in the following shot. Also in the shot before that line she's pointing the fossil at Ross, but when it cuts to a front view she's pointing it upwards.
Factual error: In season 4 generally, Phoebe has the shortest pregnancy ever. She gets pregnant as the boys win the girls' apartment. In the next episode Ross meets Emily, after 6 weeks he gets engaged to Emily and they then choose to get married 4 weeks later. Phoebe would only be 10-12 weeks pregnant by the time they fly to London, not in her third trimester.