The One With All The Rugby - S4-E15
Stupidity: Emily tries to help Ross inflict pain on the other rugby players and tells him about one player who doesn't wear a cup. No one who knows rugby would ever wear a cup, it is more hazardous than helpful to wear a cup in rugby as it involves a lot a grinding, and a simple slip or movement could cause serious damages to the male genitals. Especially if these guys are British and well versed in rugby which they are.
The One With All The Resolutions - S5-E11
Stupidity: When Ross is in the bathroom, there is literally a towel right next to him and he doesn't use it.
The One With Frank Jr. - S3-E5
Stupidity: The friends discover that Joey's built a ridiculously oversized entertainment center only after they've pushed it upright. Cute concept, but there's no way that's when it would have become obvious. First of all, all they had to do was see it stretched across the doorways when it was lying flat on the floor. Second of all, Chandler had been watching it get built; even if he wasn't really paying attention, it would have been very immediately obvious at a single glance. And Joey's a space cadet, but there's no way he wouldn't have noticed either.
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.