The One with the East German Laundry Detergent - S1-E5
Character mistake: In the laundromat, there is a sign where the word clothes is misspelt cloths. (00:16:00)
The One with the East German Laundry Detergent - S1-E5
Character mistake: Janice enters Central Perk with a large shopping bag and even shows Chandler what she bought but when she leaves a few hours later, she leaves without it.
The One with the Birth - S1-E23
Character mistake: Outside the room in the corridor where Carol is waiting to deliver her baby, the poster outside giving directions says "Nusery", not "Nursery". (00:02:15)
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