The One Where Chandler Doesn't Like Dogs - S7-E8
Factual error: When Monica and Chandler come back from Ross' apartment, Chandler opens the door and Monica appears from stage left when the stairs are stage right. (00:06:00)
The One Where They're Up All Night - S7-E12
Factual error: In the opening scene, Ross remarks on the stars the group is gazing at. In reality, there would be far too much light pollution in this part of New York City to be able to see any celestial bodies such as comets or stars.
The One With Ross's Library Book - S7-E7
Factual error: When Ross shows Chandler the book, he claims it is in "the biggest library on campus" at NYU, which would be Bobst library on Washington Square Park South. Later, in Central Perk, when Ross tells Rachel and Joey about the people "making babies" in front of his book, Joey says, "Wait a minute. Fifth floor, against that back wall?" Bobst's fifth floor is conference rooms only; there are no stacks on that floor. Joey might have been confused, but Ross should know better, being a professor and having just visited that location. (00:08:30)
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.