The One With Christmas in Tulsa - S9-E10
Continuity mistake: After Chandler says "Merry Christmas" to the gang, they all respond - Phoebe just goes "blargh blargh", and has her hands in her lap. They all turn to look at her, and suddenly her hands are resting on the sides of the stool. (00:10:30)
The One With Christmas in Tulsa - S9-E10
Continuity mistake: When the group notice that it's snowing outside, external shots show a lot more snow than the internal shots. (00:19:20)
The One With Christmas in Tulsa - S9-E10
Audio problem: In the scene before the opening credits, if you listen carefully to Chandler's line, "If I don't get it done, I'll be fired", you can tell it's an overdub. The background noise and volume levels are different than the rest of his speaking. (00:00:33)
The One With Christmas in Tulsa - S9-E10
Continuity mistake: When Wendy is coming onto Chandler her hands are at the top of his tie. When the shot changes, her hands are halfway down his tie.
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