The hotel where the Friends stay in this episode, the London Marriott, was the actual hotel that the cast and crew stayed in while filming.
Lisa Kudrow was pregnant in real life during the filming of this episode, and was not able to travel to London. The writers had to find a way to keep her involved in the story while still in Monica's apartment.
When Rachel and Phoebe are discussing if Rachel loves Ross, Phoebe's necklace switches sides constantly between shots.
In the very beginning of the scene where Phoebe and Rachel get the call from Joey, a crew member can be seen walking out of the shot on the left.
When Rachel is telling Phoebe she's going to London, she comes out of her room with a sweater over her shoulders. It shows Phoebe and two seconds later it shows Rachel again, but the sweater is gone. Then she goes to the wall and grabs a jacket. It goes back and forth between Rachel having the sweater and not having it.
When Phoebe is talking on the phone with Emily's mother and making a sandwich, she puts mayo on her sandwich. But at the end of the conversation there is no mayo on her sandwich.