Revealing mistake: In the episode where Joey is doubling Al Pacino's butt you can clearly see his shadow when he's shooting the scene in the shower, and he's wearing boxer shorts. (00:13:20)
Revealing mistake: After Chandler asks the woman out, he runs back over to the group shouting, "She said yes! She said yes!" As he does this Joey can be seen mouthing the line. (00:03:45)
Revealing mistake: When Joey is filming his scene for the movie, look behind the director to see a box saying 'Monica' indicating that it was shot on the Friends set rather than Al Pacino's set. (00:16:30)
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