The One With Rachel's Crush - S4-E13
Audio problem: In the scene where Rachel walks into the guy's new apartment and they start talking about Joshua, it cuts to a shot of her in profile as she says "It's just so frustrating". Her mouth doesn't move as she says the words. (00:09:40)
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