The One With The Engagement Picture - S7-E5
Revealing mistake: In the shot of the paper with the picture of Monica and Joey, the anouncement written there starts out fine, but near the end the middle part is just repeated again. (00:20:35)
The One Where Chandler Doesn't Like Dogs - S7-E8
Revealing mistake: While Monica is sneaking the dog back to her apartment and Chandler is in the toilet, as Monica lifts her hand to her mouth she whispers, "Up!" to get the dog to stick its head out of the handbag. (00:15:00)
The One Where Chandler Doesn't Like Dogs - S7-E8
Revealing mistake: When Rachel and Tag are in the hall at the end of the episode, the tan mark from Jennifer Aniston's wedding ring is visible on her hand. (00:19:15)
The One With All The Candy - S7-E9
Revealing mistake: At the start of the episode, when Ross says, "No, one daddy, two mommies," Ben is mouthing the line along with him. (00:00:15)
The One With the Holiday Armadillo - S7-E10
Revealing mistake: When Ben sits on the sofa to listen to Ross talk about the festival of lights, you can see Monica move Ben to the other side of the sofa despite there being space for her to sit. This is so they sit in the correct order as she would mask Ben to the cameras.
The One Where They're Up All Night - S7-E12
Revealing mistake: Just after Ross realises that he and Joey are locked out on the roof, he goes over to the door and tries the handle with some force. That whole part of the set moves - door and walls. (00:03:25)
The One Where They're Up All Night - S7-E12
Revealing mistake: When Chandler is getting a pan out to warm some milk, he successfully pulls the bottom pan out without too much disruption, however, the joke (and entire plot device - to wake Monica up) is to make as much noise as possible so Perry pulls out more pans on purpose and knocks them on the floor. This is quite clear - you can see him grasp the wok and pull it out onto the floor. (00:06:30)
The One With The Truth About London - S7-E16
Revealing mistake: When Monica gives Phoebe the headache medication (Hexadrin) she, not only doesn't take the lid off - and therefore fails in pretending to put it back on - but also doesn't actually give Phoebe anything. Extended episode. (00:07:30)
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 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 With Chandler's Dad - S7-E22
Revealing mistake: In the shot of the highway with Ross and Rachel in the Porsche before the police officer walks to them, it is very obvious that the people inside the car are not actually them.
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves the apartment and Chandler stays listening to the answering machine, check the wall beside the door. It changes the angle, as if somebody moved it (probably to hide the hall).
The One With Monica and Chandler's Wedding, Part 1 - S7-E23
Revealing mistake: When Monica leaves her apartment in the scene where she thanks Chandler for staying calm since they got engaged you can see the wall on the left side of the door moving as the door closes.
The One With Monica and Chandler's Wedding, Part 2 - S7-E24
Revealing mistake: At the point in the ceremony where Joey forgets the rings, the camera pans out and you see Chandler step sidewards to get the ring from Ross, and Monica step sidewards to get the ring from Rachel and Phoebe. As Monica moves, her dress slides across and one of her feet is revealed; it has what looks like a brown boot or heavy brown shoe on it. (00:20:05)
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