The One With The Cheap Wedding Dress - S7-E17
Continuity mistake: Monica chooses a wedding dress at an expensive boutique, then later goes to a discount store to buy the same style dress at a half-off sale. The dress she buys is different from the one in the boutique, having wider and frillier straps. When she tries it on later at the apartment, it is now the same style as the one at the boutique. (00:04:00 - 00:07:15)
Revealing mistake: When everyone leaves the apartment together for the last time, they show Chandler and Monica with the twins in the stroller, and you can tell they are dolls.
The One With The Baby Shower - S8-E20
Visible crew/equipment: Monica chases after Rachel's Mom to apologise and she falls over and you can clearly see the safety mat move when Monica falls on to it - it's blue and just down the first step. (00:22:00)
The One With The Mugging - S9-E15
Revealing mistake: When Rachel tells Joey that his agent called and he got the audition for the Broadway play, Rachel's wearing a colorful paisley top, and after Monica says, "Maybe the monkey will be at the audition," in the next shot it's Jennifer Aniston's stand-in, wearing a red plaid blouse, who is standing beside Joey. (Widescreen). (00:03:08)
The One With The Proposal, part 1 - S6-E24
Continuity mistake: In the scene where Richard is being seated next to Chandler and Monica, the amount of champagne in Chandler's glass changes. (00:09:20)
The One With Rachel's Other Sister - S9-E8
Other mistake: When Joey and Phoebe are talking while Rachel and Amy are fighting, Phoebe says the line 'Oh my God. Shouldn't we stop this?', Joey is mouthing her lines. (00:20:25)
The One In Barbados, part 2 - S9-E24
Other mistake: The layout of the hotel surrounding the main cast's rooms as displayed during the sequence where Monica, Chandler, and Phoebe eavesdrop on Ross, Charlie, Rachael, and Joey is physically impossible. Ross enters the room Monica, Chandler, and Phoebe are in through the same wall they are using to listen in on Charlie and him, though while the camera is in Ross and Charlie's room, no door can be seen in that wall. Furthermore, a hallway can be seen through the doorway while Ross holds the door open, implying that the two rooms do not share a wall in the first place.
The One With All The Wedding Dresses - S4-E20
Visible crew/equipment: When it's Monica's turn to catch the bouquet she and Phoebe switch places, and just as Phoebe asks her if she's ready, in the wideshot studio lighting rigs can be seen at the top of the screen.
The One Where Ross Got High - S6-E9
Visible crew/equipment: After Ross and Monica blurt out each other's secrets to their parents, when Jack says, "And we kind of figured about the porch swing," in his closeup we can see the end of the set wall, with the soundstage behind it and many tape marks on the floor.
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: Rachel and her sister are in Rachel's room. When Rachel picks up Emma, Emma has nothing in her arms, but in the next shot when Rachel is holding her, she has a toy in her arms though she had no time to get one.
Continuity mistake: The view from Monica's apartment changes after the first few episodes.
The One With a Chick. And a Duck. - S3-E21
Continuity mistake: When Ross is asking Joey and Chandler which suit he should wear for the TV thing the brown suit is in his right hand and the blue one is in his left hand. After Ross says "really?" the shot changes to a wide view, the two suits have changed places, and the cover on the brown suit has switched to the blue one. (00:09:50)
Continuity mistake: The Chick changes breeds and gender a couple of times. It starts out as a full size White Leghorn. During the adolescent phase it is a female White Silkie. This is a bantam or small size chicken. In maturity it is a male White Leghorn.
The One With The Fertility Test - S9-E21
Continuity mistake: When Rachel takes her gift certificate to the massage parlour to claim her free massage, she hands it to the receptionist and we can see that it has been torn up into eight pieces and taped back together. Yet when Phoebe tears up the gift certificate in a previous scene, we see her tear it into only four pieces before handing it back to Rachel. (00:05:00 - 00:08:20)
The One with the Blackout - S1-E7
Revealing mistake: When Rachel first meets Paolo she's looking for the cat in candlelight, holding the candle in front of her. You can see her shadow on the wall, as well as the shadow of the candle, revealing that the light is not coming from the candle. (00:07:25 - 00:14:30)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Rachel loses the ring near the beginning, Chandler and Joey get off a chair to help look. But then the shot cuts to Monica, and behind her you can see Chandler at the fridge without enough time to get there. (00:05:40)
Audio problem: Any scenes outside Central Perk always sound as if they're on a set (which let's be honest, they are) - the sound is often muted and footsteps on the pavement have a very slight echo to them.
Visible crew/equipment: In the scene where Chandler is talking to Rachel and Ross about how Janice thought that he had somehow called her fat, if you watch Monica's closet in the back, you can see the shadow of someone walking past, under the door. (00:05:20)
The One With Ross's Inappropriate Song - S9-E7
Visible crew/equipment: When Monica comes home at the end, as Joey pulls her down on top of Chandler as to protect her eyes from seeing the tape, you can see a knee pad on Courteney Cox's knee, under her skirt. (00:14:20)
The One With The Halloween Party - S8-E6
Continuity mistake: When Phoebe is talking to Ursula's fiance Eric, he is wiping sweat off his face with a napkin. He puts the napkin to his eye and then in the next shot it changes to the other eye without him having enough time to change it. This happens again when at one point he's wiping his eye and then suddenly his forehead. (00:07:45)
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