Continuity mistake: The hallway outside Ross' new apartment (series 6 onwards) always changes. Sometimes, the apartment is at the end of a dead-end corridor, and there is a plant outside, covering up the wall; sometimes you can go past the apartment and turn left or right; or, as in this episode, you can only turn right immediately after you have passed the apartment.
Other mistake: Monica and Chandler's wedding is May 2001 (we are given the dates by the characters on the show, so it doesn't matter when the show aired.) And Rachel reveals at the wedding that she is pregnant, so that means she got pregnant in April or so of 2001. "The One With the Rumor" aired and is set to take place at Thanksgiving 2001, during which time Rachel should be 7 months pregnant. But her stomach's flat as a pancake.
Continuity mistake: When Ross comes into Chandler and Joey's apartment to ask the guys and Rachel what they are doing in four weeks, the Magna Doodle on the door behind him is blank. However, when he tells them in the same scene that Emily always wanted to get married where her parents did, the Magna Doodle says 'get out'. No character had any chance to change it.
Continuity mistake: In the final scene Rachel passes out copies of her draft for her book. She gives out all her copies and asks for their opinion. However, near the end of the scene after waving her arms around empty handed a copy of her script magically appears in her hands.
Factual error: Joey always grabs his back when he gets his pain from his kidney stones. This is a classic referral pattern for pain when the stones are in the kidney. However, the doctor says that the kidney stones were too close to the bladder. Stones in the ureters classically refers the pain to the groin area.
Continuity mistake: Ross and Joey are discussing Joey writing his own moviel; Ross sits down to begin writing, and Joey places a beer on the table next to him. The large part of the label is facing to the side, but it then switches to facing the front without being touched.
Continuity mistake: When Joey is interviewing the potential roommate and Chandler comes in, Joey's stance changes a few times. His grip on the application form changes, and when he points to the bathroom his arm is in a slightly, but noticeably, different place in the next shot.