The One with the East German Laundry Detergent - S1-E5
Continuity mistake: While Rachel and Ross are at the laundromat there's laundry detergent, softener, bleach, etc., atop the soda machine, some of which change position or vanish in consecutive shots.
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Rachel and Ross are in the apartment with Marcel, who's still wearing the pink tutu, the bottle of wine can be seen on the side table beside the teapot, but when Ross remembers the wine it's now between the teapot and the sofa.
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Monica takes the newsletter from Rachel it's open to the pages with engagement news, and we can see the left page has pictures as Monica reads Mindy and Barry's notice on the right page, but when Monica wiggles out of her comment about Mindy being pretty that left page is now only words at the bottom, even though she's still open to the same page with Mindy and Barry's notice.
The One with George Stephanopoulos - S1-E4
Continuity mistake: When Ross wrestles his puck away from the little kid in the emergency room, it goes flying at the receptionist. The shot cuts, and it hits her from a completely different angle.
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: In the scene where Phoebe confesses to the gang that Paolo made a pass at her, it is pitch black outside. When Joey and Chandler turn around after heading for the door, it is sunny outside.
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: When Ross says he could escape from any uterus, Joey is holding two small books. In the next shot they have vanished and he is holding a large book. (00:02:10)
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)
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 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 the Blackout - S1-E7
Continuity mistake: In the scene where Joey is standing with candelight over Ross' neck, the candle is full length, but in the next shot it's only half length. (00:13:00)
Continuity mistake: In the scene just before Joey goes into the shower, when he is thanking the director for the opportunity, the camera shot goes back and forth between Joey and the director several times. During this series of shots, the part in Joey's hair switches sides nearly every time. It appears that the entire shot is reversed - in the closeup, the shower curtain is on his left and the shadow to his right is sharply focused. In the lower (long) shot, the curtain is to his right, and the shadow on his left is more focused. (00:13:05)
The One with Two Parts: Part 1 - S1-E16
Continuity mistake: When Chandler is playing with the toy in his office, his right hand moves between shots. (00:09:35)
The One with the Thumb - S1-E3
Continuity mistake: When Phoebe visits Lizzie, Lizzie is feeding the pigeons with her right hand. In the next shot, a plastic bag has appeared in her left hand from nowhere. (00:11:40)
The One with the Stoned Guy - S1-E15
Continuity mistake: When the stoned guy eats all the shrimp ravioli, he eats everything including the garnish. When we next see the plate, the garnish has returned. (00:17:30)
The One with Mrs. Bing - S1-E11
Continuity mistake: When Joey says to Ross in the hall 'you didn't call and leave your grip size' his arms are folded. In the next shot, his arms are behind his back. (00:10:20)
The One with the Thumb - S1-E3
Continuity mistake: At the end of the episode when Phoebe asks Monica if Alan mentioned them, the towel on the table is next to Phoebe and is untidy. In the next shot, it has moved closer to Chandler and is folded neatly. (00:21:20)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: When Ross falls into the grave he ends up with a massive mud patch on his knee. He turns over and it's gone. (00:14:30)
The One with Two Parts: Part 2 - S1-E17
Continuity mistake: At the end of the scene where Phoebe goes to speak to Ursula, Ursula has some food on her fork. In the next shot it is gone. (00:16:10)
The One Where Underdog Gets Away - S1-E9
Continuity mistake: When Rachel serves Joey coffee after he comes into Central Perk wearing make up, she puts the cup down on the far left hand side of the table. As Joey picks it up, it has moved to the far right hand side between shots. (00:02:45)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: When Ross is on about calling immigration, his hands move with no time to do so. (00:04:05)
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.