The One Where Chandler Gets Caught - S10-E10
Continuity mistake: Ross claims he looks into Monica and Chandler's and watches them eating dinner and watching TV. His apartment is a floor lower down than theirs and with the balcony wall, the sofa and tables distance from the window and the fact the bottom of the window is higher than the floor, there is no way Ross would be able to see that far in. We have seen in the past that he can basically only see who is at the window. (00:19:05)
The One Where Chandler Gets Caught - S10-E10
Continuity mistake: When Phoebe calls Chandler in the car he is not wearing the seat belt. The shot changes and suddenly he is wearing the seat belt. A second later the seat belt is off again.
The One Where Chandler Gets Caught - S10-E10
Continuity mistake: After Chandler says he loves the house, Monica says "this is huge" with her arms by her sides. We then cut to a front shot, and Monica's holding her hands in front of her. (00:04:30)
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