The One With All The Wedding Dresses - S4-E20
Continuity mistake: Ross comes into Chandler and Joey's apartment to ask the guys and Rachel what they are doing in four weeks, the Magna Doodle behind him first says "poop", then its screen is blank. We cut to Joey checking his diary, cut back to Ross, and it now has "get out" written on it. [Fullscreen version]. (00:10:35)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: Rachel takes a drink from the Jones Soda and sets the bottle down. A second later, the bottle is fuller than it was to begin with. (00:10:05)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: When Chandler tells Joey that he should go to a sleep clinic, Monica puts her coffee cup on the table, but then it reappears in her hands and disappears repeatedly. (00:02:00)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: When Chandler is telling Monica and Phoebe about the girl who picked him over Joey, look behind him and you will see Gunther nodding to a lady in a denim jacket. In the next shot, the woman walks up to the counter again, and again Gunther is nodding at her. (00:17:00)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: When Rachel suggests to Joshua that they should get married, he says "What?", and his hands are out in front of him. The camera cuts to a wide shot and his hands are now on his thighs. Cut back to Joshua and his hands are now out in front of him with his fingers interlocked.
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