The One With The Stain - S8-E7
Plot hole: When Monica falls off the chair you can see that she is wearing a white bra because you can see some of the cup and the strap (when she says "you're not getting it". Towards the end of the episode (still the same day) we find out that Monica is meant to have the same bra on as their new cleaner, Brenda. The bra she is meant to have on is 'pink with the lacy cup'. Rachel notices that Monica is wearing that bra and she shows Monica. What happened to the white bra she had on a few scenes before? (00:11:15 - 00:19:10)
The One Where Underdog Gets Away - S1-E9
Continuity mistake: Chandler is able to get into Monica's apartment to tell everyone that Underdog got away, but later when Rachel closes the door it is now locked. We can see the door as each person is leaving and nobody does anything which could be construed as locking the door before it closes. It's been often demonstrated in the show that the door doesn't lock automatically. (00:13:20)
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 George Stephanopoulos - S1-E4
Continuity mistake: Just before Ross gets hit in the hockey game, you can see him wearing a foam finger which then disappears. (00:11:45)
The One Where Ross Finds Out - S2-E7
Other mistake: When Rachel leave her message on Ross' answering machine she mentioned the name Michael after 17 seconds and "I'm over you" after 25 seconds. When Ross listen to the tape he asks "Who is Michael ?" after 8 seconds. Rachel flips the phone out of his hand after 18 seconds so he couldn't have heard her "I'm over you." (00:10:35 - 00:12:50)
The One With The Rumor - S8-E9
Continuity mistake: The gang confronts Ross over his affair with a fifty-year old librarian when he was younger. If you watch the mirror behind him (in one of the girls' rooms), it changes angles every time the camera cuts between Ross and the rest of the gang. (00:17:40)
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)
Continuity mistake: When Pheobe is talking to Ursula's stalker, her hands move places with no time to do so. (00:02:45)
Continuity mistake: Just before Chandler gives Monica the mixed tape, he tries to manipulate a coat hanger into something feasible. He stretches it out and holds it up, saying "Yes, honey, I made it myself," then the camera cuts to another angle as he throws it down. After the cut the hanger is not as stretched as it was before. (00:10:40)
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 All The Thanksgivings - S5-E8
Continuity mistake: When Phoebe's trying to make the TV work at the beginning, in the close-ups and the first wide shot the remote's at an angle, but in the second wide shot it's straight. It then goes back to being at an angle. (00:00:15)
The One Where The Stripper Cries - S10-E11
Visible crew/equipment: At the end, when Ross and Chandler are arguing about what bed Rachel was on (in the flashback), a line of stage lights is visible in the copper pans behind them. (00:21:40)
Continuity mistake: Joey's Hand moves from putting it to his mouth to the plate instantly at breakfast. (00:06:35)
The One with the Ick Factor - S1-E22
Continuity mistake: Monica is in her apartment with her boyfriend and they are kissing, and she invites him to stay the night. Watch the glass of wine she has poured herself: the amount of wine in the glass keeps changing. (00:08:45)
The One With Ross's Sandwich - S5-E9
Continuity mistake: In Central Perk, when Phoebe writes a note for Ross to put on his sandwich so no one eats it, she mentions that she stabbed a cop, and has a green mug in her hand. It cuts to Monica then back to Phoebe and the mug is out of her hand and she's yelling, "He stabbed me first." When it cuts back to the wide shot the mug is back in Phoebe's hand. (00:05:40)
The One With All The Thanksgivings - S5-E8
Continuity mistake: When Monica puts the sunglasses and hat on the turkey, she puts the glasses on where the arms are over the wings of the turkey. But in next shot the arms of the glasses are completely under the wings. (00:21:10)
The One With The Football - S3-E9
Visible crew/equipment: After Joey's initial conversation with the Dutch woman, as he is walking away speaking to Chandler on his way back to the game, you can see the shadow of the camera is on his chest and moves away as he walks. (00:08:30)
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 The Rumor - S8-E9
Continuity mistake: Right near the end of the episode when Joey comes in wearing the maternity pants, he pulls out the chair from the table twice. (00:21:20)
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)
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.