The One Where Chandler Can't Cry - S6-E14
Visible crew/equipment: While Rachel's looking out Monica and Chandler's window she tells Joey that Ross is on a date with her sister, and just as Joey comments, "I didn't know we could date your sister," a crew member can be seen moving around near the front door at the left side of the screen, and we know it's not Chandler or Monica since they're still in the bedroom wearing robes in the next shot.
The One Where Chandler Can't Cry - S6-E14
Continuity mistake: When they're watching the vampire video, Monica, Chandler, and Rachel are sitting on the couch, but in the closeups of the TV we can see the couch in the reflection and it's empty.
The One Where Chandler Can't Cry - S6-E14
Continuity mistake: In this episode Chandler says he can't cry, but in series 4 when he broke up with Kathy he was seen crying, and other times - it's just this episode it's apparently an issue.
The One Where Chandler Can't Cry - S6-E14
Continuity mistake: In the beginning of the episode when Joey tells Chandler that he hasn't forgiven him for kissing one of his sisters, Monica and Chandler's TV, entertainment set and ferns are missing from the set and there are cables protruding from the wall. The area is completely bare except for the French poster. Everything's back to normal slightly later in the episode.
The One That Could Have Been, part 1 - S6-E15
Revealing mistake: In the opening credits for both parts, fat Monica jumps on the couch and it lifts up on Chandler's end. If you look carefully at Chandler, you can see him holding one end of the couch with his hand and pressing up with his foot. (00:02:10)
The One That Could Have Been, part 1 - S6-E15
Continuity mistake: Near the start, Chandler is sitting in the coffee house in the big chair. When he's talking to Joey about money, his feet change positions every two seconds. On the floor, crossed, resting on the table, the list goes on and on. (00:04:55)
The One That Could Have Been, part 1 - S6-E15
Continuity mistake: In the scene where Joey gives Chandler the orange juice and says sorry, the paper cup's logo faces Joey's hand, then the shot goes to Chandler and when it comes back, the logo on the cup faces Chandler. (00:15:15)
The One That Could Have Been, part 2 - S6-E16
Continuity mistake: When Phoebe is in the hospital bed and Monica gives her a cup of herbal tea, Phoebe is shouting at her for putting honey in it. As the shot changes back and forth the spoon in the cup changes position. (00:02:00)
The One That Could Have Been, part 2 - S6-E16
Continuity mistake: Rachel is talking to Joey in his apartment after waking up the next morning, looking messy and rumpled. As she talks, her bra strap slips down her arm. In one shot (as she says "I'm a horrible person"), the strap is all the way down her arm and then in the next shot it is tucked under her shirt. (00:10:00)
The One That Could Have Been, part 2 - S6-E16
Continuity mistake: When Chandler goes into Monica's apartment after he beeps Dr Roger for a "ruptured spleen" his hair is sticking up through the front of his cap. After he tells Monica "You should be with me", the shot goes to Monica and when it goes back to him, his hair is neatly tucked into the cap.
Revealing mistake: At the very beginning, as Gunther complains to Joey about talking to his friends, he then says something like, "This guy has been waiting for his coffee for ten minutes". He passes a mug to Joey, but it is very clear the mug is empty. (00:01:40)
Continuity mistake: In the beginning in the coffee house, Ross is explaining to Phoebe and Rachel what 'Unagi' is. The blonde lady behind Phoebe is wearing light blue jeans. She then puts a brown leather jacket on, however in the next shot it has gone, only to reappear and disappear again. (00:03:20)
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)
Visible crew/equipment: When Chandler points out to Joey that his oblivious comment was made in front of Ross it cuts to a closeup of Ross, and camera equipment can be seen at the right side of the screen, and it's still there in the next two closeups of Ross.
The One Where Ross Dates a Student - S6-E18
Continuity mistake: The fireman's helmet alternates being under each arm between shots. (00:05:10)
The One Where Ross Dates a Student - S6-E18
Visible crew/equipment: When Joey walks into Chandler's apartment, Chandler is sitting on the couch talking on the phone. Two microphones are visible. (00:06:20)
The One Where Ross Dates a Student - S6-E18
Continuity mistake: When Monica is standing in front of Phoebe, asking if the smell from Joey's is bothering her, the shots of Monica from the front show her arms held out in front of her, but when the shot changes her arms are crossed. This happens twice. (00:11:15)
The One Where Ross Dates a Student - S6-E18
Continuity mistake: When Chandler, Joey and Ross are talking about Donna Keystone from college, Ross is holding a water bottle; it disappears, then he is holding it again. (00:13:15)
The One Where Ross Dates a Student - S6-E18
Continuity mistake: In the dinner scene between Chandler and the girl from college, the spoon in front of Chandler moves without him touching it. (00:13:45)
The One Where Ross Dates a Student - S6-E18
Visible crew/equipment: Just after Ross' girlfriend leaves the Central Perk, Ross walks towards the couch. Just before Rachel starts talking, the stage lights can be seen reflected in the glass above the entrance. (00:17:00)
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.