The One With The Embryos - S4-E12
Continuity mistake: At the end, when Monica asks about the garbage bags the apartment's front door is closed, but when Rachel opens the drawer and freaks out about what's inside, the front door is wide open (for the camera) as she kicks the drawer shut, and then the door is closed again. (Widescreen).
The One With The Embryos - S4-E12
Continuity mistake: When the friends are talking about the contest to prove who knows each other better, Monica says to Joey "Are you scared?" and Rachel has her arms crossed. Joey says "No" and in the next shot of Rachel she is crossing them again. (00:05:00)
The One With The Embryos - S4-E12
Continuity mistake: When Ross takes a bite of the cookies, he says they are not good, and puts it back in the box; in the next wide shot he is seen putting it back in the box again. (00:03:15)
The One With The Embryos - S4-E12
Continuity mistake: In the last scene, Chandler is sitting on the couch in the new apartment, and the rooster is sitting on the back of the couch, with his tail facing the window. When Joey comes running out, the shot of the rooster shows his face toward the window, and then in the next shot he has returned to his tail facing the window. (00:20:45)
The One With The Embryos - S4-E12
Continuity mistake: The part in Chandler's hair switches from left to right throughout the episode.
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.