The One With Ross's Teeth - S6-E8
Character mistake: While on his date with Hillary, Ross says that he has a five year old son. Ben was born in 1995 and this episode is set in 1999, so Ben should be four, not five.
The One Where Ross Hugs Rachel - S6-E2
Audio problem: In the pre-theme-song scene, with Phoebe and Ross talking outside the coffeehouse, when Phoebe sends Rachel away in the cab for the second time the sound of the tyres screeching starts while the cab is still stationary.
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