Revealing mistake: In the one after Monica and Chandler get married, Chandler says he can't dance because his shoes are slippery. If you look closely, he managed to slip so much by standing on the bottom of Monica's dress, not because of the shoes. (00:06:00)
The One with Two Parts: Part 1 - S1-E16
Revealing mistake: If you look closely you can see that the name of the restaurant "Riff's" is printed on by computer in post-production. (00:00:05)
The One With Chandler's Dad - S7-E22
Revealing mistake: When we first see the Porsche, there are smear marks on the bonnet from previous takes of Ross diving across it. (00:04:35)
The One With Joey's Big Break - S5-E22
Revealing mistake: When Joey and Chandler are traveling in the car, the background is a very obvious projection. (00:11:45)
The One With Chandler In A Box - S4-E8
Revealing mistake: When Chandler is in the box and Ross accidentally places his hand over the 'air-hole' you can see Ross feeling for the hole for a while before he finds it. (00:12:35)
The One After Ross Says Rachel - S5-E1
Revealing mistake: When Phoebe phones Emily's step-mother to say she is Ross' doctor, she looks out the window behind Phoebe. The buildings are moving slowly from side to side. (00:04:10)
The One With Rachel's Inadvertent Kiss - S5-E17
Revealing mistake: When Ross is doing his "Watching TV" bit, there is a brief shot of the blank screen that he is sitting on the sofa pretending to watch. In the screen's reflection, it is clear that the sofa is empty. (00:03:55)
The One Where Underdog Gets Away - S1-E9
Revealing mistake: Whenever Monica's apartment is shot from the point of view of the balcony looking in, fake walls have to be put up across the room to cover the open space (since there is no wall there usually). When the friends are looking out the window at Ugly Naked Guy and his girlfriend in this episode, you can see that the rear wall does not extend nearly far enough - there is a large black space behind Monica. (00:19:25)
Revealing mistake: At the very beginning, as Gunther complains to Joey about talking to his friends, he then says something like, "This guy has been waiting for his coffee for ten minutes". He passes a mug to Joey, but it is very clear the mug is empty. (00:01:40)
The One With The Cooking Class - S8-E21
Revealing mistake: When Ross confronts Rachel saying are you sure you don't want more, she gets up saying she has to pee - as she stands up, she turns her back to the camera, and you can she the buckle for her mum-tum on her back. (00:20:35)
The One With The Cake - S10-E4
Revealing mistake: When Rachel and Ross are on the way back, if you look at Ross you can see a purple tinge to the side of his face. This is from projecting the background. (00:17:30)
The One Where Estelle Dies - S10-E15
Revealing mistake: When Chandler and Monica are at the door of the house you can see that it is a fake background behind Chandler. (00:09:35)
The One With All The Thanksgivings - S5-E8
Revealing mistake: When Joey has the turkey on his head, and Chandler is by the door, look at the wall to Chandler's right. It's a "wild wall" - i.e., a movable wall put there to extend the set outward an additional few feet, since the camera normally doesn't see that far forward. The wall angles oddly away from the room, and you can see how the line and pattern of the hallway floor continues into the room. (00:06:15)
The One With The Bullies - S2-E21
Revealing mistake: In the scene where Ross, Chandler, and the bullies are fighting in the street, an extra bumps the fire hydrant and it wobbles and slides over. (00:21:55)
The One The Morning After - S3-E16
Revealing mistake: In the shot where Monica and Phoebe are taking off the wax strips, they make a big fuss about how much it hurts. However, watch carefully and you will see one of the strips just fall off. (00:10:30)
The One In Barbados, part 2 - S9-E24
Revealing mistake: When Ross is talking to the weird obsessive scientist at the bar, just before the wife comes in, the wall behind the professor moves in shot to allow the cameraman room to swing around in the next shot. (00:40:20)
The One Where Eddie Won't Go - S2-E19
Revealing mistake: When the men are repossessing Joey's things, a view of New York can be seen through the window, but it's clearly a picture. (00:08:00)
The One With Chandler's Dad - S7-E22
Revealing mistake: The number (licence) plate on the Porsche changes between the time it is outside the coffee shop (YSO 791) and the time Rachel and Ross are driving on the highway (JGL 334). (00:04:35 - 00:13:00)
The One After Ross Says Rachel - S5-E1
Revealing mistake: When Chandler and Monica (and later Joey) are talking on the plane on the way back from London, the little display screen (showing a map of the Atlantic and coastlines) in front of Monica's seat shows the little grey plane indicating their position and direction. The plane is facing east, as though they are travelling towards London, but they are supposed to be flying back to New York. (00:14:45)
The One With The Soap Opera Party - S9-E20
Revealing mistake: When on the roof for the party, the view of the city is obviously fake. (00:12:20)
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