Continuity mistake: Rachel and Monica are talking about Phoebe getting pregnant at the table. Rachel's hands change every time where the camera is showing them in a different angle.
The One Where Rachel Tells... - S8-E3
Continuity mistake: In a deleted scene, the security guard opens Monica's case. There is a blue item on top, when the camera changes, this item has disappeared.
The One With Joey's Interview - S8-E19
Continuity mistake: In the opening scene, Joey is holding the Soap Opera Digest magazine. In one shot it is open but folded, then in another it is open all the way, then it's folded again, then it's open again, then it's closed, and then it's folded again. The change is never shown.
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.
The One With The Birthing Video - S8-E15
Continuity mistake: When Chandler tells Rachel and Monica to look at the screen when the baby has come out, his arm goes from pointing at the TV to on his lap.
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