The One With the Sharks - S9-E4
Other mistake: While Monica, Rachel, and Joey are seated at the table talking about Joey's problem, there's a Dasani water bottle on the table with part of the brand name painted black so it reads "Asav" on the label.
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