Continuity mistake: when Monica is asking Zack if he or any of his blood relatives has diabetes, Zack's cutlery changes position twice. In the shot facing him he has his fork in his left hand and his knife in his right. When it changes to a side shot, he is gesturing and doesn't have anything in his hands, without having time to put the cutlery down. Then when it cuts back to a front shot, he picks up his fork, which is now on his right, and starts eating.
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: When Joey and Phoebe are talking about his co-star spitting on him, and Phoebe says that she gets spit on at work, Joey puts his hand on her arm. Then in the next shot, when Monica and Chandler are walking along, Joey puts his hand on Phoebe again.
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: At the end when Rachel comes into the coffee house and sees Mona with Ross, she is holding a brown paper bag. When you see her in the background, the the top of the paper bag is rolled over away from her. When the camera switches to just Rachel, the paper bag is the other way around with the top rolled over towards her.
Continuity mistake: When Monica is standing in front of Phoebe, asking if the smell from Joey's is bothering her, the shots of Monica from the front show her arms held out in front of her, but when the shot changes her arms are crossed. This happens twice.
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: When Monica and Chandler enter the apartment arguing about what time they were supposed to meet, Monica has her handbag strap over her right shoulder, and then takes it off. When the shot switches to Phoebe claiming to know what time they said they would meet, the handbag strap is back over Monica's right shoulder, and then she takes it off again.