The One With the Baby on the Bus - S2-E6
Continuity mistake: When Chandler and Joey fetch Ben from the Department of Health Services, as they say they're both the baby's father, Chandler puts his arm around Joey and Joey takes his hand. When the angle changes you can see Joey's shoulder and Chandler's hand isn't on there. When it cuts back to the front view, his arm is around Joey again. (00:15:10)
The One With Barry & Mindy's Wedding - S2-E24
Continuity mistake: Phoebe takes out her gum before kissing Joey. After she kisses him, she makes a comment about it and her mouth is empty. In the next shot, she sits down and is chewing. But she doesn't put the gum back in. (00:02:22)
The One With Ross' Step Forward - S8-E11
Other mistake: In the opening scene when Rachel says "I went from store to store sitting on Santa laps" the camera is out of focus.
The One With Phoebe's Rats - S9-E12
Continuity mistake: When Gavin and Rachel are in Rachel's office, in some shots Gavin is wearing a wedding ring. (00:03:43)
The One With the Holiday Armadillo - S7-E10
Revealing mistake: When Ben sits on the sofa to listen to Ross talk about the festival of lights, you can see Monica move Ben to the other side of the sofa despite there being space for her to sit. This is so they sit in the correct order as she would mask Ben to the cameras.
Continuity mistake: When Phoebe goes to visit her mom for the first time, she takes a cookie from the cookie jar and leaves the lid off. But in one shot, the lid is back on. The lid is back off in the following shots. (00:06:36)
The One With The Princess Leia Fantasy - S3-E1
Other mistake: When Phoebe makes fun of Ross' Princess Leia fantasy with the rolls, Rachel tries to stop her; the shots used are the same action, just from different angles. Look at Rachel's hands and the towel in her hand when Phoebe puts the rolls by her ears.
The One In Barbados, part 2 - S9-E24
Continuity mistake: As Mike and Monica are playing ping-ping, two people playing checkers magically materialize behind Monica. One second they're not there and the next second they are. (00:12:25)
The One Where Everybody Finds Out - S5-E14
Continuity mistake: When Monica and Chandler come in to ask Joey about Pheobe knowing about their relationship he throws the Hugsy teddy behind him. In the next shot we see behind the chair and Hugsy is not there.
The One With The Free Porn - S4-E17
Other mistake: 10:50pm boarding and departure, but it is broad daylight outside. (00:09:25)
The One With The Giant Poking Device - S3-E8
Continuity mistake: When Ben says Monica's name she's holding the Minute Maid juice box, but in Ben's closeup the juice box is on the table, though it's back in Monica's hand in the next shot. (00:08:50)
The One Where Monica Sings - S9-E13
Visible crew/equipment: After Joey dips his finger into the hot wax and smears it all over his hands, when Sonya walks out of the back room we can see her tape mark on the floor, which she promptly steps on.
The One Where Monica Sings - S9-E13
Continuity mistake: When Phoebe, Monica and Rachel are talking in the coffee house, a waitress walks by and in the next shot when the camera focuses on Monica, and comes back to Rachel, the waitress walks by again. (00:05:00)
The One After the Superbowl, part 2 - S2-E13
Visible crew/equipment: When Joey walks into the bathroom and starts whistling, the shadows of the camera and a passing crewmember can be seen on the door of Chandler's cubicle, then the camera's shadow moves as Joey walks toward Chandler and also when Joey's about to stand on the toilet.
The One with the Monkey - S1-E10
Continuity mistake: When David kisses Phoebe in the lab the first time, he writes "yes" on the board. Later on when they are in the lab again the "yes" is different than the previous one.
The One With Ross's Wedding, part 2 - S4-E24
Audio problem: Toward the end of the episode Ross' Dad and Emily's dad are arguing. Ross comes between them and you can hear the audio saying"From now on everybody gets along!" But it does not match up with the movement of Ross' lips.
The One Where Ross Hugs Rachel - S6-E2
Visible crew/equipment: After Ross tells Phoebe that he does not have feelings for Rachel, just as Ross walks into the apartment to find Rachel crying, the shadow of the moving boom mic can be seen on the wall at the top, left side of the screen. (00:20:15)
The One With Ross and Monica's Cousin - S7-E19
Continuity mistake: When Monica comes into Phoebe's apartment for her Surprise Bridal Shower, the view of the hallway outside the apartment is a dead end on the right side (the left side cannot be seen), however, in S7:E12 (The One Where They're Up All Night), both when Phoebe is disposing of the fire alarm and when the fireman is at the door, it is clear that there is another apartment opposite hers, as well as a trash chute on the right side. (00:18:25)
The One With The Truth About London - S7-E16
Revealing mistake: When Monica gives Phoebe the headache medication (Hexadrin) she, not only doesn't take the lid off - and therefore fails in pretending to put it back on - but also doesn't actually give Phoebe anything. Extended episode. (00:07:30)
The One Where They're Up All Night - S7-E12
Revealing mistake: When Chandler is getting a pan out to warm some milk, he successfully pulls the bottom pan out without too much disruption, however, the joke (and entire plot device - to wake Monica up) is to make as much noise as possible so Perry pulls out more pans on purpose and knocks them on the floor. This is quite clear - you can see him grasp the wok and pull it out onto the floor. (00:06:30)
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.