The One With All The Haste - S4-E19
Continuity mistake: Phoebe comes in wearing Santa pants. When Joey and Chandler point this out, Chandler's legs are crossed or uncrossed, depending on the camera angle. (00:04:10)
The One With All The Haste - S4-E19
Continuity mistake: The singing neighbor is more than halfway done tying his tie but when Rachel yells at him, the tie is merely draped around his neck.
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