The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves the apartment and Chandler stays listening to the answering machine, check the wall beside the door. It changes the angle, as if somebody moved it (probably to hide the hall).
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves her apartment in the scene where she thanks Chandler for staying calm since they got engaged you can see the wall on the left side of the door moving as the door closes.
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