The One Where No One's Ready - S3-E2
Other mistake: When Monica beeps into Richard's machine and changes the outgoing message, she always presses the same two buttons when trying to interact with his machine, but gets a different result each time. (00:19:00)
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