The One Where Monica and Richard Are Just Friends - S3-E13
Revealing mistake: When Richard gets tomato juice on his shirt and Monica goes to get some soda for it, you can't see any juice on his shirt. (00:08:25)
The One Where Monica and Richard Are Just Friends - S3-E13
Revealing mistake: When they are in the coffee house and go over to the counter to discuss Phoebe's boyfriend's exposure out of his shorts, in the long shots you can see behind that there is no way they would be able to see anything.
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