The One The Morning After - S3-E16
Revealing mistake: In the shot where Monica and Phoebe are taking off the wax strips, they make a big fuss about how much it hurts. However, watch carefully and you will see one of the strips just fall off. (00:10:30)
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