The One With The Red Sweater - S8-E2
Plot hole: They say that whoever the sweater belongs to left it behind, but in The One Where Rachel Finds Out that Ross videotaped them together it is clear that Ross is not wearing anything under the sweater, therefore for Ross to have left it behind he must have gone home topless.
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