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: 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: 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.
Continuity mistake: The view from Monica's apartment changes after the first few episodes.
Continuity mistake: The Chick changes breeds and gender a couple of times. It starts out as a full size White Leghorn. During the adolescent phase it is a female White Silkie. This is a bantam or small size chicken. In maturity it is a male White Leghorn.
Continuity mistake: Ross always had an end apartment. At some point in Season 8, his apartment became a middle apartment. In earlier season, when Joey had mistaken Ross' for the hot girl's apartment, it showed the apt as an end apt. In TOW Joey tells Rachel, when Joey asked Rachel to have dinner at Ross' apt, it showed another hallway where Joey came from which is supposed to be the wall/ Ross' kitchen.
Continuity mistake: In addition to Ross remaining 29 for several years, he also gets younger as the years go by. When he marries Emily he is thirty (Monica describes herself as "a single mother with a 30 year old son"), but when he divorces her he is twenty-nine.
Continuity mistake: In the early episodes there is a wooden post in front of the door at Monica's apartment. Remember when Monica and Rachel were looking after Ben and Monica bangs his head - Monica shouts "Monica bang" and runs into it. The post doesn't appear in later series. This was due to the post being an obstacle to shoot around.
Continuity mistake: Apparently Ross is Monica's older brother but in "The one where they're going to party", Joey states that the three men are all the same age ("we're 29, we're not women"), and in The One with the Birth Monica says she is 26 and Joey says he is 25. If Joey is younger than Monica, and Ross and Joey are the same age, then there's definitely a mistake somewhere.
Continuity mistake: The bathroom in Chandler and Joey's apartment constantly changes in the direction it opens in. Sometimes the doorknob is on the left, sometimes it is on the right. The changes occur throughout the seasons, and in some cases the changes occur in back-to-back episodes.
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: In the pilot episode, the guy Rachel left at the altar is called Barry Finkle. In all the other episodes his name is Barry Farber.
Continuity mistake: Mr Heckles's apartment number changes from 8 to 5.
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Phoebe cleanses Ross' aura, he holds up both hands to stop her. In the next shot his left hand is lowered. (00:02:50)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: The door to Monica's bedroom is shown open, then closed, then open, and then closed during the scene in which Paul arrives for their date. (00:09:00)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Monica asks Ross, 'Do you want me to stay?', her hands are in front of her, not touching Ross. In the next shot, her right hand is holding onto Ross's wrist. (00:09:10)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Paul the Wine Guy first comes to Monica's apartment, Phoebe is standing in the kitchen telling Rachel that she just pulled out four eyelashes. But in the very next shot, you can see her sitting in an armchair in the living room. [Broadcast version only - in the extended DVD version there's a shot showing Phoebe walking to the chair.] (00:09:20)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Ross shows Chandler and Joey the can of "Carol's favorite beer" it's unopened as Ross makes a crack about how he "should have known," but in the next shot we see Ross lifting the can and drinking from it, even though he never actually pulled open the stay-tab between the two shots (viewed in widescreen). [In the extended version of the episode you see Ross open the can before taking a drink. In the standard version, the mistake is valid]. (00:09:55)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Ross, Chandler and Joey are setting up Ross' furniture and Ross is bent down saying, "And I cannot feel my legs," Ross is holding up a manual. When it cuts, the manual is suddenly lying on the floor. [In other versions, such as Netflix, the first shot shows the manual falling to the floor and the second shot does not exist. However, the mistake is valid for the TV broadcast version of the episode]. (00:11:45)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Joey hides the bracket in the plant pot, the instruction sheet that is on the bookcase disappears instantly. (00:12:20)
Answer: The statements that she's contradicting (that it is common, it happens to every guy, and it's not a big deal), are the things that a woman commonly says to a man who is suffering from erectile difficulty, typically to assuage his bruised ego. However, most men do not believe that these statements are true, as evidenced by Chandler's outburst. He's so caught up in the proof that women are lying about it that he gives himself away.
Rooster of Doom