The One With The Giant Poking Device - S3-E8
Continuity mistake: When Chandler is breaking up with Janice and she walks away trying to shake him off, her jumper is untied. When Chandler says "You can't go, I have your shoe" we cut back to Janice and her jumper has been tied at the bottom.
The One With The Giant Poking Device - S3-E8
Continuity mistake: When Rachel and Monica are stressing about the bump on Ben's head, Rachel's hair keeps moving from tucked behind her ear to hanging loosely in her face between shots.
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: In the scene where Chandler is sitting on the cabinet talking to Rachel and Monica, Rachel is holding an ice-cream bucket and a spoon. However, they keep showing up on the counter and then back in her hands between shots.
The One With The Dirty Girl - S4-E6
Continuity mistake: Rachel is working on the crossword puzzle and her legs are crossed (first left over right) and they switch back and forth between shots (about four times) without any time to do so.
The One With The Inappropriate Sister - S5-E10
Continuity mistake: When Joey and Phoebe are talking to each other on the couch, Rachel tucks her hair behind her ears, but in the next shot it's moved.
The One With The Inappropriate Sister - S5-E10
Continuity mistake: When Joey is talking to Ross about him being an actor, the shot changes and Ross is holding his beer in a different position.
The One With The Inappropriate Sister - S5-E10
Continuity mistake: When Rachel and Monica are talking about Danny and the girl on the subway, Rachel has tiny bits of her fringe hanging out and it moves between shots without any movement.
The One With a Chick. And a Duck. - S3-E21
Continuity mistake: At the beginning of the episode, Phoebe gives Monica a push on her rollerskates and then we cut to Gunther giving Rachel a mug of cocoa. At this point, Monica is about to crash into Rachel, but as the angle changes we see Phoebe giving Monica a push for a second time.
The One with Two Parts: Part 2 - S1-E17
Revealing mistake: When the scene cuts from the apartment to Riff's, a man on the other side of the street walks past the Riff's sign and the sign is still visible through him, revealing it is just animated.
The One with Mrs. Bing - S1-E11
Revealing mistake: When Chandler finds out Ross kissed his mom, he leaves and slams the door behind him. After he slams the door, the wall wobbles slightly, revealing it's part of the set. (00:15:05)
The One with Mrs. Bing - S1-E11
Continuity mistake: The position of Mrs. Bing in relation to the pay phone keeps changing during her conversation with Ross at the restaurant. (00:08:55)
The One with Two Parts: Part 2 - S1-E17
Other mistake: Rachel is limping in the hospital for the first time. When she gets back home, she walks perfectly fine. Then in the hospital, she starts to limp again.
The One With Princess Consuela - S10-E14
Continuity mistake: When Ross hugs Rachel in Central Perk, after finding out she has been fired, he leans his face against hers twice.
The One With Ross's Library Book - S7-E7
Continuity mistake: When Phoebe is eating the muffin, it has no white paste on it. Then the shot changes and there's a line of white paste.
The One With Ross's Library Book - S7-E7
Continuity mistake: After Monica tells Chandler that they are going to have lunch with Janice, it shows Central Perk. The people in the window are different when the shot changes.
The One With Ross's Library Book - S7-E7
Continuity mistake: When Rachel asks Ross what's wrong, her hand is up. Shot changes and it's down.
The One With Ross's Library Book - S7-E7
Continuity mistake: When Janice sees Monica's ring, she's wiping her hands with a rag, then it disappears. (00:03:50)
The One Where Paul's The Man - S6-E22
Visible crew/equipment: In the scene where Rachel is trying to hide Ross from Paul, when he's about to go into Paul's bedroom after he says "Did you really hear a bear?", look at the window behind Rachel when the shot is on her. You can see a large light reflected in the glass and it moves slightly (not just in conjunction with the camera, it moves before the camera shifts).
Visible crew/equipment: When Ross and Rachel realise they are in an empty apartment, studio lighting can be seen over their heads in the ceiling as Ross leads Rachel into the centre of the room. (00:04:35)
The One Where Chandler Doesn't Like Dogs - S7-E8
Visible crew/equipment: When Chandler sees the empty dog basket on the floor at Ross' there is a panning shot to the door. If you look carefully, you can see a crewmember wearing blue jeans and white shoes. (00:17:25)
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.