Other mistake: When Phoebe and Ross are at JFK, some of the people who are at that airport are the same people who are also at Newark. It's particularly funny to see the people behind Ross and Phoebe at JFK when Phoebe claims they're on their honeymoon, and when it cuts to Rachel at Newark as she's looking for her boarding pass, those same people are there too.
Revealing mistake: When everyone leaves the apartment together for the last time, they show Chandler and Monica with the twins in the stroller, and you can tell they are dolls.
Continuity mistake: When Phoebe's driving Ross to the airport in her cab the rear view mirror vanishes from the windshield, then it reappears and disappears again. Additionally, when they start their drive the cab doesn't have a side view mirror on the passenger side, but en route the cab has acquired one, then it vanishes again.
Other mistake: The photo in Rachel's passport at the airport is not of her. (00:29:45)
Continuity mistake: When the friends put down their keys, the keys' positions change when the camera pans the apartment. (00:25:30)
Factual error: When Rachel is boarding the plane to Paris, the plane's right hand side is being used for boarding. However in all airlines all over the world, they always use the left side for boarding. The right side is a 'service' side.
Continuity mistake: On the plane, when Rachel says "She's almost never right," (about Phoebe) you can see the guy's hand is on the armrest. Then a split second later, his hand is about half a feet off the armrest while saying "But she is sometimes." (00:15:15)
Other mistake: When Joey sat in the paint the first time, there was a mark, but the second time, it didn't leave a mark.
Continuity mistake: After Ross decides to go after Rachel, in the overhead shot all four of the cab's doors are closed, but when it cuts to Ross and Phoebe getting in and shutting their front doors suddenly both back passenger doors are inexplicably wide open, then when Ross shouts about missing seatbelts both back doors are closed again. Then when the guy gets into the back seat he shuts his door, but when he asks for the medallion number both back doors are open, and then they're closed again.
Revealing mistake: When Phoebe and Ross hit heads while in the cab on the bridge, they only just tap heads. Not enough to make them scream in pain. (00:05:15)
Other mistake: When Rachel attempts to board the plane the first time, the man at the desk looks at her passport upside down.
Character mistake: Even though both Ross and Rachel have cell phones with them and use them for other calls, it never occurs to Ross early on and as he's frantically racing to the airport with Phoebe, to call and tell Rachel that he loves her or at least to say that he is on his way to tell her something important before her plane takes off for Paris.
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.