The One Where Ross Is Fine - S10-E2
Continuity mistake: When Owen is telling his parents Chandler paid him $50 not to tell them he knows he is adopted, Chandler says "which technically now you should give back" and he is looking at Owen. In the next shot his back is to Owen.
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: When Ross is looking at Emma in the coffee house he turns and looks at the guys on the couch and mouths the word "what." Then in the next shot he does it 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 The Cake - S10-E4
Continuity mistake: When Jack and Judy leave Joey's apartment, there is a birthday banner on the counter that has fallen off. When Monica, Chandler, Joey and Phoebe are racing the toys to see who gets to go, the banner is back on. In this time no-one has fixed the banner as there is not enough time for them to do so off camera and the people on camera don't fix it. It's most clear that it's back on when Monica is saying, "Technically, our dog never crossed the finish line". (00:14:05 - 00:17:05)
The One Where The Stripper Cries - S10-E11
Continuity mistake: When the stripper first walks in, he is wearing a uniform with a circular badge. A few moments later the badge is not circular, it is a shield type and shape and the uniform appears to be different.
The One With Ross's Grant - S10-E6
Continuity mistake: When Phoebe tells Rachel and Monica that Mike lets her keep her box of hair, Rachel's chopsticks are down, by the plate. In the next shot, she's holding them close to her mouth.
The One With Ross's Tan - S10-E3
Continuity mistake: Chandler walks away from Ross's apartment at the end. But he walks to where the kitchen should be. (00:21:15)
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.