The One Where the Monkey Gets Away - S1-E19
Continuity mistake: While Rachel's babysitting Marcel she's watching the soap opera on TV, and when she points out Dr. Francis there's a closeup of the TV and when she points out Raven there's another closeup, but the TVs in both closeups are different, as well as the items that are near the TVs.
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: After Marcel has escaped from the basement, he is eating a banana as a pair of arms reach out to grab him. These arms are supposed to belong to Mr Heckles, their weird neighbour, however the arms belong to someone young, large and hair free. (00:16:10)
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Rachel and Ross are in the apartment with Marcel, who's still wearing the pink tutu, the bottle of wine can be seen on the side table beside the teapot, but when Ross remembers the wine it's now between the teapot and the couch.
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Monica takes the newsletter from Rachel it's open to the pages with engagement news, and we can see the left page has pictures as Monica reads Mindy and Barry's notice on the right page, but when Monica wiggles out of her comment about Mindy being pretty that left page is now only words at the bottom, even though she's still open to the same page with Mindy and Barry's notice.
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Rachel tells Ross that she "kind of lost" Marcel it cuts to an exterior shot from the balcony, and if we look past Rachel and Ross we see there's a table against the wall, under the Maïna La Voyante poster, but that table does not exist in previous or following shots. (00:10:15)
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.