The One Where Monica and Richard Are Just Friends - S3-E13
Revealing mistake: When they are in the coffee house and go over to the counter to discuss Phoebe's boyfriend's exposure out of his shorts, in the long shots you can see behind that there is no way they would be able to see anything.
The One With Frank Jr. - S3-E5
Other mistake: Monica doesn't realise what the floor looks like under the hamper, but due to her obsessive cleaning she would know this for sure.
Factual error: The makeup on Rachel's face "can't be covered up with makeup," and "won't come off," we learn in The One In Vegas, Part 2. So it's not a surprise that she still wakes up with it on (and so does Ross.) but an hour or so later, when she's at breakfast, it's not there. An hour or two won't make the magical difference that 8+ hours didn't.
The One Where Joey Moves Out - S2-E16
Other mistake: When Rachel is showing Phoebe her heart tattoo, the tattoo isn't dressed and covered, nor is the skin around it red or swollen, despite the fact she just finished getting it. (00:14:09)
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: In the episode before, Rachel's hair is just underneath her shoulders. Then, a few weeks later when Ross gets back from Spring Break, it's a lot longer than before.
The One Where Rachel Tells... - S8-E3
Visible crew/equipment: When Monica is yelling at the other couple on their honeymoon, the things on the shelves behind them shows the reflections of set lights. (00:15:25)
The One Where Old Yeller Dies - S2-E20
Continuity mistake: A woman and a man in a blue shirt exit the coffee shop twice.
The One With The Bullies - S2-E21
Continuity mistake: A woman and a man in a blue shirt exit the coffee shop twice.
The One With The Bullies - S2-E21
Visible crew/equipment: While driving the cab, Phoebe is wearing a blue shirt. When the car stops suddenly, a crew member wearing a white shirt is visible driving the car.
The One With The Halloween Party - S8-E6
Audio problem: When Rachel greets Mona at the door of the apartment, visually she says "Hi" but the audio is " Hi Mona. "
The One With Chandler's Dad - S7-E22
Character mistake: When Chandler says "and there's daddy", an extra behind him chuckles in time with the studio audience. He's the only one laughing and it happens right on cue with the audience.
The One With All The Jealousy - S3-E12
Continuity mistake: At the start, Ross says, "Botanists are such geeks." When he does, his left hand goes from in his lap to on the table instantly. (00:00:35)
The One Where Chandler Can't Remember Which Sister - S3-E11
Continuity mistake: When Joey is saying about ticking his Grandma off, he has his hands up in front of him. When the shot changes, his hands are lowered. (00:13:20)
The One With The Giant Poking Device - S3-E8
Continuity mistake: At the start, Phoebe snaps at Rachel that she will go to the dentist. When she says this, her grip on the apple in her hand changes between shots. (00:02:30)
The One With Ross's Inappropriate Song - S9-E7
Audio problem: When Emma laughs right before Ross comes in, the laughs don't match with her lips.
Continuity mistake: When Joey and Chandler talk about how to tell Monica that it is too soon for marriage, Joey fills his plate up with scrambled eggs. When he walks over and sits at the table, the eggs appear smaller on his plate. (00:15:54)
The One With Chandler In A Box - S4-E8
Visible crew/equipment: After Joey tells Chandler he's going to his folks and might stay there for a few days while looking for an apartment, just as Joey retrieves his keys from the shelf, a crew member wearing a black shirt and white pants is visible on the left side of the screen. (00:08:40)
The One Where No One's Ready - S3-E2
Continuity mistake: Just before Rachel and Ross are leaving Monica's apartment at the end, Rachel stops by the door but the next shot they're further in the room, then they walk past the door and out.
The One Without The Ski Trip - S3-E17
Continuity mistake: When Monica and Phoebe enter the coffee house and talk to Chandler and Ross, the extra sitting at the coffee bar (black shirt, blue jeans) is not wearing a belt, then when they pan again she is, and back she is not. (00:00:12 - 00:00:54)
The One With The Ride-Along - S5-E20
Plot hole: When Rachel picks up the box of photos they all fall out, because the box isn't sealed with tape or even folded properly. But there's no way for that to have happened. Monica would have brought the box from somewhere and put it on the couch, and the photos would have fallen out then, and she would have fixed it (this is Monica, after all.) The only way for this to have happened is if she put the empty box on the couch, unfolded/unsealed, and then put photos in it - which Monica would not do.
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.