The One Where Joey Tells Rachel - S8-E16
Continuity mistake: At the end of the previous episode, Joey tells Ross that he has feelings for Rachel just as Ross is about to walk out of Central Perk, and there's a vase of tulips on the table beside the door and two things hanging on the coat hooks. The opening scene of this episode continues at the same moment of Joey's admission, and the vase has vanished and there's only a sweater hanging on the coat hooks.
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