The One With The Halloween Party - S8-E6
Continuity mistake: While Phoebe is calling out Ursula for lying to Eric, her hair continuously changes position from behind her shoulder to in front of her shoulder depending on the camera angle. (00:14:50)
The One With the Pediatrician - S9-E3
Continuity mistake: After Rachel's 3 AM phone call to her pediatrician, when Rachel suggests using Dr. Gettleman she raises her left hand and points at Ross, but in the next closeup it's her right arm that's raised.
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: After Joey runs out of the apartment to go make the fake videotape, when it cuts to Rachel walking back into the apartment the table with the tablecloth is quite far away from the front door, but when Caitlin knocks and Ross rushes to go flirt with her that table is near the door. (00:13:35 - 00:15:15)
The One Where Rachel is Late - S8-E22
Visible crew/equipment: After Rachel has told Ross it's 100° outside and she starts walking towards the door, a camera is visible to the far right (Widescreen version only). (00:09:30)
The One With The Fake Party - S4-E16
Continuity mistake: When Emily says that she'll go first at spinning the bottle we see the door behind Ross is wide open, but after Emily spins the bottle that door is closed, yet it's wide open again during the game and then closed.
The One With The Fake Party - S4-E16
Continuity mistake: When Joey takes the steaks out of the freezer, Phoebe's holding a napkin up by her face with her left hand while her right hand is down and empty, but in the next shot it's her right hand that's up with a napkin.
The One With Joey's Dirty Day - S4-E14
Continuity mistake: After Chandler gets back from the strip club, when he throws his jacket and scarf they both land on the rug between the recliner and coffee table, but when Chandler sits in his recliner the jacket is gone and the scarf is lying on the floor on the opposite side of the table.
The One With The Girl Who Hits Joey - S5-E15
Continuity mistake: When Ross tells Chandler and Rachel that he's running out to buy name tags for his party, in the next shot the water bottle beside Rachel does not have its cap on, but it cuts to Rachel's closeup and the cap is on. Other differences can be seen as well, such as Rachel's hair and an extra utensil.
The One Where They're Going To PARTY! - S4-E9
Continuity mistake: When Joanna stops Rachel from leaving work Rachel's holding her handbag and jacket, and there's still one bag hanging on the wall hook, but when Joanna mentions the "drinking binge" there are two bags hanging from the hook, then it's back to just one bag, yet when Joanna calls Sophie there's nothing on the hook and Rachel has both bags.
The One With The Ballroom Dancing - S4-E4
Continuity mistake: While Treeger chews out Rachel for not properly disposing the pizza box in the trash chute, the room's door is closed, then wide open, and then closed again.
The One With The Apothecary Table - S6-E11
Continuity mistake: While Monica and Chandler confront Janine in Joey's apartment, we can see the drawing of the fire breathing dragon and the ape on the Empire State building on the Magna Doodle, then when Joey tells Chandler and Monica to leave because he needs to talk to Janine, the drawing differs - note the dragon's scales, its position, the building, etc.
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: While the two couples are at the restaurant, Chandler has a beer and Gary, Monica, and Phoebe have margaritas, and the amount of liquid in Gary's margarita glass keeps changing, back and forth from less to more.
The One With The Ride-Along - S5-E20
Continuity mistake: When Rachel is at Ross' apartment she gets the blender and while she's standing at the window talking to Monica on the phone the kitchen door is closed, then it's wide open, and then closed again.
The One With Princess Consuela - S10-E14
Continuity mistake: After Ross announces his tenure, when Rachel starts crying and tells him that she got fired and didn't get the other job, her right hand is up by her face, but in the next shot her right hand is holding the glass of champagne and it's her left hand up by her face.
The One With Ross's Sandwich - S5-E9
Continuity mistake: During the literature class, when they're discussing Wuthering Heights all the writing on the blackboard changes, and the word "conflict" is first spelled correctly, but then it's spelled without the letter L.
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: While everyone's watching Joey's commercial for the Milk Master 2000, the TV area changes in the closeups.
The One With The Nap Partners - S7-E6
Continuity mistake: When Ross and Joey wake from their nap on the couch while everyone else is watching them, Marvin the Martian stuffed doll is lying on the floor between the couch and wall, but in the closeups Marvin the Martian is on the couch.
The One With The Baby Shower - S8-E20
Continuity mistake: During the mock Bamboozled game with Chandler and Ross, when Joey asks about the word triskaidekaphobia a bottle of Evian suddenly appears on the countertop oven, which vanishes and then reappears.
Continuity mistake: After Ross decides to go after Rachel, in the overhead shot all four of the cab's doors are closed, but when it cuts to Ross and Phoebe getting in and shutting their front doors suddenly both back passenger doors are inexplicably wide open, then when Ross shouts about missing seatbelts both back doors are closed again. Then when the guy gets into the back seat he shuts his door, but when he asks for the medallion number both back doors are open, and then they're closed again.
The One Where Heckles Dies - S2-E3
Continuity mistake: After Monica breaks the seashell lamp, when Phoebe says that she wants to believe Monica did it accidentally her right is arm up and points to Monica, but in the next shot it's Phoebe's left arm that's up while still pointing at Monica.
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.