Continuity mistake: When Ross introduces himself to Mona, the reception place card that Mona's holding switches from her left hand to her right hand, between shots.
Continuity mistake: When Monica tells Joey and Ross she is not pregnant, her hand goes to her chest, then down to her side, and then back to her chest. (00:02:10)
Continuity mistake: Just as Ross and Mona are about to dance, her arms are crossed. The camera cuts and they are on her hips. (00:16:05)
Continuity mistake: When Rachel spits out the champagne for the first time, her necklace is center chest. When the camera cuts it is far right. (00:13:25)
Continuity mistake: When Ross says , "Yes, I am." after Mona calls him sweet, His hands are by his side. Next shot one of them is by his chest. (00:16:30)
Continuity mistake: When Monica says she wants to dance, her hair is always changing depending on the shot. (00:21:30)
Continuity mistake: Near the end when Monica and Chandler are going on to dance for the last time, Monica speaks and her hair is neatly behind her right ear. The shot then looks to Chandler before going back to Monica immediately. However her hair is suddenly behind her left ear when she can't have moved it as her hands are in the shot at all times. (00:21:15)
Continuity mistake: When Joey and Chandler are discussing Joey's shoes, the length of the sleeve round Joey's neck changes depending on the shot. (00:11:20)
Continuity mistake: When Monica and Rachel are discussing Phoebe, Rachel's hands go from playing with the table to dry washing depending on the shot. (00:12:30)
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.
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