The One with the Birth - S1-E23
Continuity mistake: In the scene when Carol is giving birth, there is a shot when Carol shouts "Where were you?" Her trousers are visible under the sheet. (00:16:25)
The One with the Birth - S1-E23
Character mistake: Outside the room in the corridor where Carol is waiting to deliver her baby, the poster outside giving directions says "Nusery", not "Nursery". (00:02:15)
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