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 Ross's Sandwich - S5-E9
Continuity mistake: In the scene where Joey has just brought home his date, he jiggles the door handle, which makes Monica and Chandler run off into Chandler's room. Chandler kicks a circle cushion off the mat, but when the camera cuts back, it's on the mat again, and then off the mat again when Rachel walks in. A red cushion also moves. (00:09:10)
The One With Ross's Sandwich - S5-E9
Continuity mistake: In the second scene with Phoebe and Rachel in the night class, when the camera looks over Rachel's shoulder, she has "Jane Eyre" in her hand face up, but when the angle switches to front view, she has nearly placed the book on the desk. (00:13:20)
The One With Ross's Sandwich - S5-E9
Continuity mistake: When Ross is in Central Perk talking about his day at work and how he is not going to work for a while because of his rage, watch his cotton candy. It changes in sizes and shape a lot, from big and uncircular to small and circular. (00:14:50)
The One With Ross's Sandwich - S5-E9
Revealing mistake: Joey's bucket of chicken is, rather obviously, a KFC bucket with "Finger Lickin' Chicken" stuck over the original KFC lettering - you can even see the picture of Col. Sanders. (00:17:10)
The One With Ross's Sandwich - S5-E9
Continuity mistake: After Joey makes up the story about how he slept with Monica once in London, in the shots facing Phoebe, Rachel, and Ross the door to Monica's apartment goes back and forth being either wide open or closed, depending on the angle of the shot.
The One With Ross's Sandwich - S5-E9
Continuity mistake: During the literature class, when they're discussing Wuthering Heights all the writing on the blackboard changes, and the word "conflict" is first spelled correctly, but then it's spelled without the letter L.
The One With Ross's Sandwich - S5-E9
Character mistake: Joey tells Chandler and Monica that Rachel found the underwear in Monica's sofa. It was actually Phoebe who pulled them from the sofa and held them up.
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.