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.
Continuity mistake: Phoebe and her mother are in her mother's house at the beach. Phoebe has just found out that Phoebe senior is her real mother. Have a look at Phoebe's mother in this episode and the last episode of series 3 (The One At The Beach). Her hair is a lot shorter in series 4 even though no time has passed in the storyline. (00:03:35)
Continuity mistake: When Chandler, Joey and Monica tell their story of the jellyfish bite Chandler says sometimes at night he still hears Joey's voice screaming "Do it ! Do it!". Joey says he yells it through the wall at night to freak Chandler out, but no nights have passed. They tell the story the same day the jellyfish bite occurred. Notice Rachel, Ross and Phoebe are wearing the same clothes they had before they left the beach house. (00:16:20)
Other mistake: When Chandler and Monica are sitting at the table after playing Gin, Chandler's bottle of Yoo-hoo is on the table, but the label reads "Yooh" because the brand name has been partially covered - the blue letters "oo" are yellow to match the background.
Continuity mistake: In the last few seconds of season 3, Ross closes the door immediately after he steps through it. However, in the opening seconds of season 4, we see this same event from the other side of the door. This time however, Ross doesn't close the door for at least 15-20 seconds after entering the room.
Continuity mistake: Joey's encounter with the encyclopedia salesman is riddled with errors. First, when the salesman is talking about Van Gogh and saying "He painted that," he points to a page that is all text; later, Joey is reading about vomit, and it is at a pont much further on in the book than when the salesman read about vulcanized rubber (which it shouldn't be, given the encyclopedia is alphabetized); finally, when Joey buys the "V" volume, as he opens it he has a $50 bill in his hand, and when the shot changes to show the salesman he is somehow folding and pocketing it without having taken it from Joey. (00:13:00)
Continuity mistake: In Joanna's office, Chandler and Rachel are pulling his pants from one another while he is cuffed to the filing cabinet. Right before Rachel says "What if I clean your bathroom for a month?" she throws the pants onto a chair, but then when Chandler later says "Freedom. I want my freedom. Why won't you hear me?" he opens the door to yell to Sophie and they are both holding the pants again. (00:15:10)
Continuity mistake: The handcuffs that Joanna uses on Chandler have a single metal bar connected via pivot to a double bar with the lock area. When Joanna first cuffs Chandler, the double bars on both sides face up at his wrists, but when Chandler's on the phone and when Rachel unlocks his right cuff, the handcuffs are going round his wrist the opposite way with the single bars facing up - quite impossible. Then the same thing occurs when Rachel handcuffs Chandler to the filing cabinet.
Continuity mistake: Rachel is telling Chandler to break off his relationship with Joanna, who had walked through the room and went into Chandler's bedroom, shutting the door. By the end of the conversation between Chandler and Rachel, the door is open but no one has entered or left the room.
Other mistake: In the scene where Rachel is going to take out the trash, Monica tells her the garbage room is the 3rd door on the left. However, when Rachel comes back with the pizza box and Monica leaves for the garbage room, she leaves the apartment and walks towards the right - there are no doors that way, just the stairs. (00:00:50)
Revealing mistake: During the end bit, before we get to see Phoebe when she is being interviewed at Lara and Jeni's Massage, if you look closely at the name of the massage parlor, you'll see that, although the camera shakes and bumps, the words stay on the very same spot on the screen. Obviously the canopy was blank and the name was digitally added later.