The One with the Fake Monica - S1-E21
Revealing mistake: In the scene when Marcel has to get on the plane. It is obvious that David Schwimmer is just shaking his leg and Marcel is doing nothing besides sit on David's foot. It is especially obvious in the second shot. (00:20:45)
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