The One With The Birth Mother - S10-E9
Plot hole: David Schwimmer is 6'2" tall and Jennifer Aniston is 5'6". There is no way that clothes that fit Rachel would also fit Ross. When he puts on the clothes that Rachel bought it's obviously not Rachel's size. If it really were, it would be a lot tighter and shorter (Ross could probably wear it belly free).
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