The One With The Cooking Class - S8-E21
Continuity mistake: The night before they go to the baby furniture store, Ross and Rachel are in the kitchen talking. Just before they leave the kitchen watch Rachel's robe: in one shot the 'lapels' are neatly one over the other, and in the next shot one lapel has moved lower and looks halfway folded over as if she hastily pulled the robe together. (00:03:15)
The One With The Cooking Class - S8-E21
Visible crew/equipment: In The One With The Cooking Class, just after Ross gets back from his date, a boom mike is hovering over Rachel's head. (00:17:25)
The One With The Cooking Class - S8-E21
Revealing mistake: When Ross confronts Rachel saying are you sure you don't want more, she gets up saying she has to pee - as she stands up, she turns her back to the camera, and you can she the buckle for her mum-tum on her back. (00:20:35)
The One With The Cooking Class - S8-E21
Visible crew/equipment: When Joey and Monica are leaving the cooking class, the teacher asks about the difference between hollandaise and béarnaise sauce, and in the next shot of Monica and Joey we can see the shadow of the moving boom mic at the top left side of the screen.
The One With The Cooking Class - S8-E21
Revealing mistake: Just as the scene cuts from the streets to the baby store, the camera wobbles slightly.
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