The One With Rachel's Other Sister - S9-E8
Continuity mistake: When Rachel tells Amy that she'll let her use the Ralph Lauren discount Amy hugs her, and the glass Amy's holding in her left hand switches to her right hand and then back to her left hand, between shots.
The One With Rachel's Other Sister - S9-E8
Continuity mistake: When Ross, Rachel, and Amy arrive at Monica and Chandler's apartment the table is set, and there are folded napkins in napkin rings on the plates at each place setting, but after Amy insults Joey all the napkins have vanished.
The One With Rachel's Other Sister - S9-E8
Continuity mistake: When Amy and Rachel are arguing in Monica and Chandler's apartment, the door behind Amy leading to the guest room changes from open to closed in almost every shot. (00:19:05)
The One With Rachel's Other Sister - S9-E8
Continuity mistake: When Chandler puts the box of china down on the chair, the lid of the box is shut, but a bit later the box is wide open.
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