The One Where Rachel is Late - S8-E22
Other mistake: Ross wants Rachel to change, but she claims that it is 100° out. Monica and Chandler got married on May 15, the day Rachel was officially pregnant. 9 Months after May would be Febuary. It would be cold out.
The One Where Rachel is Late - S8-E22
Other mistake: Rachel's water breaks, but her skirt is completely dry.
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