The One Where Chandler Doesn't Like Dogs - S7-E8
Deliberate mistake: When Phoebe is trying to sneak the dog out of the apartment, the guys say that they have noticed that the bag is moving, yet the bag doesn't move at all. (00:04:10)
The One Where Chandler Doesn't Like Dogs - S7-E8
Deliberate mistake: Monica sees the dog on her settee from Ross's apartment, which wouldn't be possible from the angle as Ross's place is lower than Monica's. (00:04:50)
The One Where Chandler Doesn't Like Dogs - S7-E8
Factual error: When Monica and Chandler come back from Ross' apartment, Chandler opens the door and Monica appears from stage left when the stairs are stage right. (00:06:00)
The One Where Chandler Doesn't Like Dogs - S7-E8
Revealing mistake: While Monica is sneaking the dog back to her apartment and Chandler is in the toilet, as Monica lifts her hand to her mouth she whispers, "Up!" to get the dog to stick its head out of the handbag. (00:15:00)
The One Where Chandler Doesn't Like Dogs - S7-E8
Visible crew/equipment: When Chandler sees the empty dog basket on the floor at Ross' there is a panning shot to the door. If you look carefully, you can see a crewmember wearing blue jeans and white shoes. (00:17:25)
The One Where Chandler Doesn't Like Dogs - S7-E8
Revealing mistake: When Rachel and Tag are in the hall at the end of the episode, the tan mark from Jennifer Aniston's wedding ring is visible on her hand. (00:19:15)
The One Where Chandler Doesn't Like Dogs - S7-E8
Continuity mistake: In this episode, it is stated that Ross does not like ice cream because it's too cold. However, back in The One Where Ross Dates a Student, you can see Ross on a date with Elizabeth eating an ice cream.
The One Where Chandler Doesn't Like Dogs - S7-E8
Continuity mistake: When Rachel and Tag are on the balcony and Tag says, "What did Joey say? I like you back?" he is standing on the balcony with one hand in his pocket and the other in front of him. As the camera angle changes, Tag is now in a different position, with both his hands behind him, leaning against the balcony before he moves forward.
The One Where Chandler Doesn't Like Dogs - S7-E8
Continuity mistake: When Chandler is looking over Ross' list (right before he calls him Magellan), he has the front sheet of the paper folded up. In the next shot, the sheet is folded down.
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.