The One With Christmas in Tulsa - S9-E10
Audio problem: In the scene before the opening credits, if you listen carefully to Chandler's line, "If I don't get it done, I'll be fired", you can tell it's an overdub. The background noise and volume levels are different than the rest of his speaking. (00:00:33)
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