The One With The Ball - S5-E21
Continuity mistake: In the interrogation room, the blinds behind Phoebe keep going from open to closed in different shots. (00:14:10)
The One With The Ball - S5-E21
Audio problem: When Rachel is trying to sell the cat and someone looks the hiss is a clear overdub. (00:16:20)
The One With The Ball - S5-E21
Continuity mistake: When Monica says "Let's go team Monica!" Joey is holding the ball; when the camera angle changes Chandler has the ball and throws it behind him for someone else to catch. (00:18:15)
The One With The Ball - S5-E21
Continuity mistake: Joey goes into the bathroom and the toilet is behind the door. In a previous episode, when Joey doesn't know about Chandler and Monica, he comes out of his room and thinks it is morning. He goes into the bathroom and Chandler pushes the door open to reveal Joey asleep on the toilet with his toothbrush in his mouth, only the toilet is now on the other side of the room, not behind the door. (00:05:20)
The One With The Ball - S5-E21
Visible crew/equipment: A stage light can be seen above Ross when he asks Rachel how much she paid for her cat. (00:04:05)
The One With The Ball - S5-E21
Visible crew/equipment: A cameraman can be seen in the TV when Rachel is on about the cat she had as a kid. (00:04:25)
The One With The Ball - S5-E21
Continuity mistake: When Rachel mentions the cat's name, Ross has the ball. The camera does an instant cut, and Joey now has the ball. Ross did not throw the ball, as you hear the ball being caught every other time. (00:04:50)
The One With The Ball - S5-E21
Factual error: There is no way that a cop and a masseuse could have afforded an apartment in Brooklyn Heights, even in the '90s. (Brooklyn Heights was unaffordable in the '80s.) "Rent control" doesn't explain it either, as a vacant apartment - which is what they are looking for - would be raised to "market rate" if a rent-stabilized tenant moved out. This can't be written off as a character mistake: Gary is not naive or uninformed, and his accent makes clear that he is a New Yorker.
The One With The Ball - S5-E21
Continuity mistake: When Rachel is outside the coffee shop trying to sell the cat, in between shots, it goes from a standing to sitting position multiple times.
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.