The One With The Baby Shower - S8-E20
Visible crew/equipment: Monica chases after Rachel's Mom to apologise and she falls over and you can clearly see the safety mat move when Monica falls on to it - it's blue and just down the first step. (00:22:00)
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