The One with the Sonogram at the End - S1-E2
Continuity mistake: When Rachel loses the ring near the beginning, Chandler and Joey get off a chair to help look. But then the shot cuts to Monica, and behind her you can see Chandler at the fridge without enough time to get there. (00:05:40)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Rachel asks Phoebe if she is a twin, she is wiping the table next to her. In the next shot Rachel has her hands together. (00:11:40)
The One with the Sonogram at the End - S1-E2
Continuity mistake: At the start of the episode when Chandler is talking about Pink Floyd, his legs are both on the table. When the shot changes, his right leg is resting on his left knee. (00:00:50)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Monica holds the glass pan of lasagna up over her head to try and find Rachel's ring, when it cuts to the next shot the lasagna is still in the air, but it's different - note burned corners, etc. (00:06:10)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Ross arrives at Monica's apartment, both Ross and Monica walk past the beam in the middle of the room. In the next shot as they continue walking they walk past the beam again. (00:07:05)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When we first see Ross at the museum, the woman he works with is holding a rock. In the first shot, she is holding onto it with her right hand at the bottom and just before the shot changes she turns away from Ross. In the next shot, she is facing Ross again and her right hand has moved on top of the rock. (00:02:05)
The One with the Sonogram at the End - S1-E2
Continuity mistake: At the very start of the episode, Monica holds up both of her hands. In the next shot her right hand is wrapped around her legs. (00:00:05)
The One with the Sonogram at the End - S1-E2
Continuity mistake: Toward the end of the episode, the exterior shot of Central Perk shows about 8-10 people inside the cafe, at the coffee bar, at the tables. But the next interior shot shows the cafe is empty except for our six Friends, lounging in the closed cafe while Rachel cleans up. (00:12:15)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Monica is asking if Ross has any stories to tell, she is looking at Ross and grinning. When the camera cuts, her expression has changed massively. (00:12:00)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Barry is explaining how he is an orthodontist, he is holding a dental tool up. Camera cuts and now it's lowered. (00:19:30)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Monica has her elbow on Ross' hand, her fist is under her chin. The camera changes to a different angle and now her hand is no longer under her chin and to the side of her jaw. (00:12:00)
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.