The One With The Halloween Party - S8-E6
Other mistake: When Monica is saying "I can't answer that, Chandler's my husband" after Joey asks her who would win in a fight between Ross and Chandler, the camera is fluttering in and out of focus. (00:11:11)
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