Continuity mistake: When Rachel finds out that Ross and Julie are getting a cat, watch Rachel's neck. When she says "together?" she's not wearing a necklace; cut to Ross and Julie for a fraction of a second, then when it cuts back to Rachel saying "both of you?" she's suddenly wearing a necklace. It then disappears again when she says "isn't that just lovely?" hanging off the side of the tray she's holding. (00:05:15)
Continuity mistake: When Joey is counting the floors of Ross' building (the first time) to find out where Hot Girl lives, he is wearing a black top with a zip. When he rushes over to Ross' building he walks down the hall and knocks on what he believes to be Hot Girl's door. In these shots he is wearing a dark purple button up shirt. As soon as Ross answers the door, Joey is wearing the black top with a zip again. (00:03:15)
Continuity mistake: In season 1 ("The One With George Stephanopoulos") the date's October 20th, and Ross says his birthday "was 7 months ago". In season 4 ("The One With Joey's New Girlfriend") Gunther is pretending to collect birthdays, and Ross starts saying "mine's December the..." before being cut off. In season 9 ("The One Where Emma Cries") Ross says his is on October 18th. (00:15:10)
Continuity mistake: When Joey goes to Monica and Chandler's apartment, he goes to the fridge to get something to drink to keep his bladder full for the audition. When he closes the fridge he has two bottles in his left hand. The camera cuts to Monica, then cuts back to Joey. He then has two cans in his left hand and is holding them differently. (00:14:55)
Continuity mistake: Monica is sitting in the living room with all her wedding presents, waiting for Chandler to return. She begins to open a large parcel and reveals a brown cardboard box. Then the camera moves to Joey, and when it moves back, the parcel is wrapped up again. This happens about three times. (00:05:05)
Continuity mistake: The number of the flats near the start of the first series was 4 and 5. After a few shows they changed to 19 and 20. [The producers realised that the numbers 4 and 5 were for apartments on the lower floors and had shown Monica's apartment to be much higher up so they changed the numbers - it's still a mistake but there's the reason.]
Continuity mistake: Across Series 3 - 5, Ross remains the same age (29) for all three years. In Series 3 he tells Chandler, "No thanks, I'm 29" when offered chocolate milk; in Series 4 when the guys try to party without Gandalf they all say they're 29; and in Series 5 when he's trying to reconcile with Emily, he says either he does, or he would get divorced for the second time before age 30.
Continuity mistake: In the coffee house, when Ross and Joey are having a fight about Joey's proposal to Rachel, Ross does the "quote gesture" with his hands and Joey says he doesn't know what this means. In TOW the Secret Closet, Joey used it himself when talking to Chandler, in the scene where he asks him to bring a bobby pin to pick the closet's lock. (00:08:55)
Continuity mistake: During this episode, Chandler is having major problems slipping people dollar bills. He asks Monica if Richard (her ex) use to slip people notes. But in Series 2 there was an episode where Chandler and Joey were doing impressions of Richard; they impersonate him by slipping money to each other with no problems. So not only has Chandler forgotten that Richard used to slip money, he's also lost the ability to slip money himself. (00:05:05)
Continuity mistake: When Monica is trying to cheer Joey up, she tells him that she straightened out his shower curtain so it wouldn't get mildew. But in a previous episode (The One Where Eddie Moves In), Joey gives the gang a tour of his new apartment; the shower is clearly visible in the bathroom, and it has a door. (00:18:10)
Continuity mistake: Chandler and Monica's wedding day is May 15th 2001, as mentioned on the show. In Series 9, some time in January 2003 (as it is after the characters have celebrated Christmas and New Year), it is established that Rachel has been on maternity leave for five months, which means she took her leave in August. Since she realised she was pregnant in May the year before, when Monica and Chandler got married, this means her pregnancy lasted from April/May 2001 to August/September 2002, a time span of 16 to 17 months. This cannot be explained by saying that episodes don't have to take place in the month they air, as we are given the dates by the characters on the show.