The One Where Chandler Takes a Bath - S8-E13
Other mistake: In the opening scene, while Joey's eating the bowl of Frosted Flakes the box is on the counter, and the brand name "Kellogg's" has been painted blue with its white line extended and covered with a sticker. At the end of the episode, when Joey's eating the Cocoa Puffs the box on the counter has the brand logos for General Mills covered with stickers on the front and sides.
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