The One With The Proposal, part 2 - S6-E25
Continuity mistake: Right after Monica says, "You wanted it to be a surprise", the camera pans to Joey who shuts the door firmly, however in the next shot you can see the door gliding back open and then shutting again. (00:18:40)
The One With The Proposal, part 2 - S6-E25
Visible crew/equipment: After Joey fibs and tells Chandler that Monica went to her parents, when Chandler and Joey walk into the apartment, in the shot facing the hallway we can see some of the camera equipment past the doorway, and in the next wideshot of the apartment as the camera pans to Monica the overhead studio lighting rigs can be seen at the top of the screen.
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