The One Where Phoebe Hates PBS - S5-E4
Continuity mistake: In the opening scene where Rachel is checking the mail and is walking over to Monica's room, she has a pair of pumps on, then once she has walked in on Monica and Monica runs after her to say she was taking a nap, Rachel is wearing a pair of backless high-heeled sandals, then when Joey and Ross come in, Rachel is wearing the pumps again (we see this when Ross says "I had to talk loud because the movie was loud"). (00:00:10)
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)
The One With The Ultimate Fighting Champion - S3-E24
Continuity mistake: After Rachel and Phoebe see Bonnie and Ross in Central Perk, Rachel is yelling at Phoebe that she needs to break them up. At first, Phoebe is holding her coat in her left hand and a purse in her right, but then she is holding them both in her left, with no time to have changed hands. (00:16:30)
The One With The Video Tape - S8-E4
Continuity mistake: When Ross walks into Joey's apartment after everyone watched the tape, you see him come in and they cut to Rachel and you hear a door close, they cut back to Ross and the door is still open and he shuts it again. (00:20:00)
The One With The Tiny T-Shirt - S3-E19
Continuity mistake: When Chandler calls Joey a "big big freak", Joey's grip on the beer bottle changes from around the middle of the bottle to the neck. (00:18:00)
The One With Frank Jr. - S3-E5
Continuity mistake: When Phoebe is talking to Monica while Frank Jr is doing karate on the balcony, the blue cup in front of her moves, disappears and is back again. (00:10:20)
The One With All The Candy - S7-E9
Continuity mistake: When Ross is trying to convince Phoebe to try to learn to ride her bike again, the bike is completely covered by a large white sheet in the corner of Phoebe's room; however, in a later shot, the wheel of the bike is visible. (00:13:15)
The One With Chandler's Dad - S7-E22
Continuity mistake: When Ross and Rachel are out in the Porsche it is dark outside, but when it later cuts back to Joey in his apartment stealing Rachel's underwear it is broad daylight. It is also light outside when Joey and Phoebe are talking at the coffee house later on. (00:10:30)
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.