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: Ross comes into Chandler and Joey's apartment to ask the guys and Rachel what they are doing in four weeks, the Magna Doodle behind him first says "poop", then its screen is blank. We cut to Joey checking his diary, cut back to Ross, and it now has "get out" written on it. [Fullscreen version]. (00:10:35)
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: While Joey's on the phone with Monica telling her the phony story that he and Phoebe smell gas coming from her apartment, on the table are the glass casserole dish and lid from the chicken parmigiana and a water bottle, but in the closeup there's only a ceramic blue bowl filled with eggplants and squash, and it keeps switching in the following shots,.
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)