The One With Ross's Thing - S3-E23
Revealing mistake: When the car catches fire you can see that the fire is not in the car but behind the car. (00:05:00)
The One With Ross's Thing - S3-E23
Other mistake: When Ross goes to see Guru Saj, he has his 'thing' removed. However, it is really obvious that Guru is not going anywhere near the thing, because you can see that Ross' shirt is still tucked into his trousers, and the Guru just rubs his hands along the shirt. (00:16:35)
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.