The One In Barbados, part 1 - S9-E23
Revealing mistake: On a number of occasions in this and the next episode, the beach and the water can be seen in the background, through hotel windows. White foam is visible in a number of places, like the crests of waves, but not only do they not move while the camera is on them, they are always in the same place in subsequent shots, revealing that this is a painted backdrop. (00:26:40)
The One In Barbados, part 1 - S9-E23
Continuity mistake: When Monica and Phoebe are talking about David, the ornaments in Monica's glass change between shots. (00:11:30)
The One In Barbados, part 1 - S9-E23
Factual error: Ross' computer is a Powerbook G4 which is totally immune to PC only viruses - Chandler opened up an infected email that contained the Anna Kournikova virus. (00:12:45)
The One In Barbados, part 1 - S9-E23
Visible crew/equipment: After Monica and Phoebe leave, David and Chandler are still at Central Perk and when David comments that Phoebe is still hung up on Mike the camera can be seen at the right side of the screen.
The One In Barbados, part 1 - S9-E23
Plot hole: This is actually for both parts of "The One in Barbados": it's shown that Monica's hair gets all frizzy from the humidity. And yet she's shown all throughout with her hair out and loose in all its frizzy glory, though it bugs her. I could maybe buy that not one of the women have hair gel with her (not bloody likely.) or that the fancy hotel doesn't sell hair gel (possible). But resourceful Monica would have thought of something to calm down her hair, or she would have just tied it up in a ponytail or bun - she does care a lot about her appearance. It makes no sense other than for comic value. (00:09:30)
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.