Continuity mistake: In the scene when Joey and Chandler come into the cafe to say that Marcel got into San Diego zoo, the man who wants him for animal fighting turns and says that Ross is making a big mistake. At this point the monkey starts to go down Ross's arm and almost drops off but in the next shot he is far up on his shoulder again.
Continuity mistake: The One with Monica's Thunder is supposed to be the same night as the previous episode which is the last episode of season six, The One with the Proposal, part 2. Yet, some major things have changed that shouldn't have. Rachel's' hair is about a foot shorter, Monica's hair is quite a bit darker, and Chandler has magically lost like 20 pounds or so.
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.