The One With The Ride-Along - S5-E20
Plot hole: When Rachel picks up the box of photos they all fall out, because the box isn't sealed with tape or even folded properly. But there's no way for that to have happened. Monica would have brought the box from somewhere and put it on the couch, and the photos would have fallen out then, and she would have fixed it (this is Monica, after all.) The only way for this to have happened is if she put the empty box on the couch, unfolded/unsealed, and then put photos in it - which Monica would not do.
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