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The One After the Superbowl, part 2 - S2-E13
Continuity mistake: Chandler is talking to Susie (Julia Roberts) and a man is standing behind Chandler in a doorway. There is a cut to Susie, but when the camera goes back to Chandler the man is not there. Then he appears again. (00:03:20)
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The One After the Superbowl, part 2 - S2-E13
Continuity mistake: When Susie is asking Chandler to wear her undies, her hand is on his neck. In the next shot it is gone, and then it is back again. (00:08:45)
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The One After the Superbowl, part 2 - S2-E13
Visible crew/equipment: When Joey walks into the bathroom and starts whistling, the shadows of the camera and a passing crew member can be seen on the door of Chandler's cubicle, then the camera's shadow moves as Joey walks toward Chandler and also when Joey's about to stand on the toilet.
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.