The One Where Chandler Doesn't Like Dogs - S7-E8
Revealing mistake: While Monica is sneaking the dog back to her apartment and Chandler is in the toilet, as Monica lifts her hand to her mouth she whispers, "Up!" to get the dog to stick its head out of the handbag. (00:15:00)
The One Where Chandler Doesn't Like Dogs - S7-E8
Revealing mistake: When Rachel and Tag are in the hall at the end of the episode, the tan mark from Jennifer Aniston's wedding ring is visible on her hand. (00:19:15)
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