The One With All The Resolutions - S5-E11
Continuity mistake: Rachel mentions "the big thing," so Joey sticks his fingers in his ears to block her out. When Rachel pulls on his arms, the only things on the table are Joey's bottled drink, crayons, and coloring paper. In the next shot, we see Rachel's arms and she's not holding anything. It cuts to the next shot facing Joey, as he says, "Well then, it wouldn't be a secret, so yeah, that would be OK," and now there's a drinking glass on the table. Rachel removes it in the next shot.
The One With All The Resolutions - S5-E11
Continuity mistake: At the start, when the countdown hits 3-2-1, there's a closeup facing the TV, and in the TV's reflection the couch and armchair are both quite empty, but in previous and following shots both have people seated.
The One Where Rachel is Late - S8-E22
Continuity mistake: At the end, when Rachel and Ross leave for the hospital, Rachel grabs the keys. In the close-up, when Rachel asks, "Do you think we have time?" she holds the keys up in her left hand while Ross holds her empty right hand. It then cuts to a medium shot, and the keys are now in Rachel's right hand, the same hand Ross holds. (00:20:40)
The One Where Rachel Tells... - S8-E3
Continuity mistake: At the start, before Monica and Chandler leave for their honeymoon, note the chairs around the kitchen table. When Rachel walks in to tell the couple to have a good honeymoon, the chair in front of the fridge and the chair in front of the stove have switched places.
The One Where Rachel is Late - S8-E22
Other mistake: Rachel's water breaks, but her skirt is completely dry.
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.