The One With Joey's Fridge - S6-E19
Continuity mistake: The handle to the fridge door is on the left, so when it opens the inside is visible. In earlier episodes the handle was on the right and it opened out towards the room, blocking the inside from view. (00:00:05)
The One With Joey's Fridge - S6-E19
Continuity mistake: When Joey pushes Ross against the fridge, a box falls from the top of the fridge onto the shelf beside it. A few seconds later when Joey turns to Chandler, we can see over his shoulder that the box is back on the fridge. (00:08:55)
The One With Joey's Fridge - S6-E19
Revealing mistake: In the closing scene where Ross and Elizabeth are dancing on the beach, you can see that the sky in the background is actually a picture, and you can see where it ends. (00:21:20)
The One With Joey's Fridge - S6-E19
Visible crew/equipment: After Ross nervously sputters Professor Friesen's name as "Fee-sen-stin-linder" when he walks in on Ross with Elizabeth, it then cuts to Ross, Chandler, and Joey walking into Joey's apartment, and we can see the opening in the wall where the bulletin board usually hangs.
The One With Joey's Fridge - S6-E19
Visible crew/equipment: In the scene where Rachel introduces Sebatian to Chandler, Monica and Phoebe, there is a visible boom mic at the top of the screen in the wide shot, right after Chandler says, "Turns out he is kinda funny".
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.