The One with Two Parts: Part 2 - S1-E17
Revealing mistake: In the scene where Phoebe goes to see Ursula at Riff's, Ursula goes to get Phoebe her birthday present. When she sits down, you can easily tell, by looking at all the people there, that the shot has been sped up. (00:15:10)
The One with Two Parts: Part 2 - S1-E17
Revealing mistake: When the scene cuts from the apartment to Riff's, a man on the other side of the street walks past the Riff's sign and the sign is still visible through him, revealing it is just animated.
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