The One After the Superbowl, part 2 - S2-E13
Revealing mistake: Rachel grabs a jar of sauce from the shelf and says, "You give me back my sweater or it's handbag marinara." You can tell from the sound it makes when it opens - no pop - that the jar had already been opened before she pulls the lid off.
The One With Phoebe's Dad - S2-E9
Continuity mistake: When Ross gives the super the money and asks "will this help with the knob" the super says that the place isn't open until Tuesday. When he says that the money is in his right hand. Then when he asks "am I not saying it right", the money in his right hand is gone.
The One With Barry & Mindy's Wedding - S2-E24
Revealing mistake: When Chandler finds out that his online girlfriend is married, the camera cuts to Joey, and you can see the computer screen is off.
The One With The Chicken Pox - S2-E23
Factual error: Ryan is a LCDR submariner, who had been on a sub deployment for the last two years. but at no time does he wear his dolphins pin. A true submariner would sooner forget his skivvies than his pin.
The One With The Lesbian Wedding - S2-E11
Revealing mistake: The first time we see Rachel's mom in the coffee house, Rachel puts her coat on the hook by the door. When she does this the wall visibly shakes.
Audio problem: At the start of the episode, when Monica is reading the review in the newspaper, you hear her say 'turn to page 153', but if you look at her mouth, she actually says a number ending with 5.
The One Where Ross and Rachel...You Know - S2-E15
Plot hole: Rachel says she didn't want to wear her glasses on their first date. Rachel doesn't wear glasses.
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.