The One With The 'Cuffs - S4-E3
Continuity mistake: The handcuffs that Joanna uses on Chandler have a single metal bar connected via pivot to a double bar with the lock area. When Joanna first cuffs Chandler, the double bars on both sides face up at his wrists, but when Chandler's on the phone and when Rachel unlocks his right cuff, the handcuffs are going round his wrist the opposite way with the single bars facing up - quite impossible. Then the same thing occurs when Rachel handcuffs Chandler to the filing cabinet.
The One With All The Wedding Dresses - S4-E20
Visible crew/equipment: When Ross asks Rachel how she feels about him marrying Emily, several stage lights can be seen in the reflection of the TV during the entire scene.
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: During the scene where Chandler is playing hide and seek with the duck and the chick, the duck moves from facing the opposite way to him, facing the same way, and then the opposite way as Joey walks in. (00:13:35)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: When Chandler is chasing Cathy, the mustard on his shirt changes place, size and darkness throughout the scene. (00:11:45)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: When Felicity finds Joey outside the wedding venue she closes the door. But when they start kissing, the door is completely open. (00:35:50)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: Chandler is chasing Kathy down the street trying to get her attention. During a close-up of him running, just before the camera cuts away we see a guy's face on the left; when it cuts back to Chandler he runs for a while and then encounters the same guy (the dog-walker). Also, when he finally lands in front of Kathy, there is a bike rider coming towards them, but as the camera cuts back and forth between the two, the rider never passes them. (00:11:30)
The One With The Dirty Girl - S4-E6
Visible crew/equipment: When Rachel says, "And not a coupon for an hour of Joey love," the stage lights are reflected in the tv. (00:04:50)
The One With The Embryos - S4-E12
Continuity mistake: In the last scene, Chandler is sitting on the couch in the new apartment, and the rooster is sitting on the back of the couch, with his tail facing the window. When Joey comes running out, the shot of the rooster shows his face toward the window, and then in the next shot he has returned to his tail facing the window. (00:20:45)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: When Chandler is telling Monica and Phoebe about the girl who picked him over Joey, look behind him and you will see Gunther nodding to a lady in a denim jacket. In the next shot, the woman walks up to the counter again, and again Gunther is nodding at her. (00:17:00)
The One With Rachel's Crush - S4-E13
Audio problem: In the scene where Rachel walks into the guy's new apartment and they start talking about Joshua, it cuts to a shot of her in profile as she says "It's just so frustrating". Her mouth doesn't move as she says the words. (00:09:40)
The One With Ross's Wedding, part 1 - S4-E23
Continuity mistake: When Monica, Ross and Emily are going to see the church, Emily's hands move between shots with no time to do so. (00:05:30)
The One Where Chandler Crosses The Line - S4-E7
Continuity mistake: When Chandler is showing Joey all of the new stuff in the apartment, the cuff on his right sleeve keeps going from messy to neat between shots. (00:13:45)
The One With The Jellyfish - S4-E1
Audio problem: When Phoebe is knocking on her sister Ursula's door to tell her about finding her birth mom, the third knock is heard a split second before you see Phoebe's hand action. (00:12:15)
The One With All The Haste - S4-E19
Revealing mistake: When they are using cards to bet on the Knicks tickets, watch Joey carefully as he goes to reshuffle the pack from behind the arm of the chair. It's obvious that there are two decks, and he is in fact switching them over. (00:13:40)
The One With Ross's Wedding, part 2 - S4-E24
Other mistake: As the wedding begins, the first bridesmaid, Felicity, walks down the aisle with a man. He is not part of the wedding party but is wearing the same attire as the best men. He is not seen again throughout the wedding. The only two men in the wedding party are Joey and Chandler. (00:38:20)
The One With The Embryos - S4-E12
Visible crew/equipment: When Rachel is protesting that she loves her apartment, look very carefully towards the top of the screen on the long shot of Monica and Rachel before it zooms in and you will see the shadow of a microphone move swiftly across the wall. (00:17:40)
The One Where Chandler Crosses The Line - S4-E7
Continuity mistake: When Ross and Phoebe are talking in Central Perk, Phoebe's position changes. (00:14:45)
The One With The Embryos - S4-E12
Audio problem: When Rachel is banging on the door at the start, the sound does not match the actions. (00:01:00)
The One With The Embryos - S4-E12
Other mistake: Ross makes up the questions for the quiz. There are two groups of questions, one for the girls and one for the boys. Each one has four categories; there are four questions in each category. From these 16 questions, Ross asks ten. Thus, there have to be six left after the quiz. But in one group, seven questions can be seen (there are seven cards left). (00:12:50)
The One With Rachel's New Dress - S4-E18
Continuity mistake: In The One With Rachel's New Dress, she's in her underwear on the sofa when Joshua comes in. In wide shots focusing on Joshua she's quite laid back, but in close shots of her she's sitting more upright. Worse than that, her legs are crossed, but uncross in the shot of his parents coming in, only to re-cross in the next shot when she gasps. (00:14:35)
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