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 extended version of the pilot, 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.
Other mistake: In the first shot of Central Perk in "The One with the Monkey" and "The One with the Dozen Lasagnas", the same footage has been used twice because the same three people are having the same conversation in both episodes.
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