The One In Massapequa - S8-E18
Revealing mistake: During Jack and Judy Geller's party, there are many shots where Elliott Gould's (Jack Geller) stand-in is perfectly visible. First, when Catherine walks over to offer congratulations to Rachel and Ross. Second, when Ross looks directly at his "dad" to ask if they have to pretend they're married. Then, numerous wide shots of "stand-in Jack" and Judy seated together in the background, such as after Rachel mentions the ring being the size of her fist, and during the toasts, etc. (00:06:00)
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