The One Where Underdog Gets Away - S1-E9
Visible crew/equipment: When Joey walks into Central Perk and everyone's laughing about the VD ads, just as Phoebe comments that laughing can be "infectious" we can see the camera at the right side of the screen.
The One with George Stephanopoulos - S1-E4
Visible crew/equipment: When Monica climbs out the window with the pitcher of Tiki Death Punch while Rachel and Phoebe are on the balcony, we can see the end of the set wall inside and a crew member moving around back there.
The One With The Apothecary Table - S6-E11
Visible crew/equipment: When Joey and Janine come back from the second double date with Chandler and Monica, just as Joey closes the door the end of the set wall and soundstage can be seen at the right side of the screen.
The One With The Routine - S6-E10
Visible crew/equipment: As soon as Janine gets up to leave the coffee house, a bright stage light with a camera and camera man right below it can be seen. (00:04:00)
The One In Barbados, part 1 - S9-E23
Visible crew/equipment: After Monica and Phoebe leave, David and Chandler are still at Central Perk and when David comments that Phoebe is still hung up on Mike the camera can be seen at the right side of the screen.
The One Where Joey Dates Rachel - S8-E12
Continuity mistake: When Phoebe flips out after losing the game and starts her cursing rant, Ross and Ben came in by opening the front door, but in all of Phoebe's closeups the front door is closed behind her.
The One With Rachel's Date - S8-E5
Continuity mistake: While Phoebe's in the kitchen with Monica and she's introduced to Tim the sous chef, the glass jar labeled "Monica's jar" suddenly appears on the shelf near the door, and it continues to vanish and reappear.
The One Where Rachel Tells... - S8-E3
Visible crew/equipment: When Phoebe asks Treeger not to mention the incident with the door to Monica and Chandler, just as Treeger uses the measuring tape we can see the opening in the wall beside the door.
The One Where No One Proposes - S9-E1
Visible crew/equipment: When Mr. Geller, Ross, and the others are looking at Emma lying in the bassinet in the nursery, in the shot facing the adults as the camera zooms in we can see the edge of the set wall (with clouds and stork) behind them, and the set lighting is visible at the top right corner of the screen.
The One With The Cake - S10-E4
Visible crew/equipment: After Rachel announces that Phoebe and Joey will perform something, Phoebe asks Joey if he'll be nervous because he has nothing prepared, and when Joey replies that he does it every day with three cameras pointed at him, it's perfect timing that in the next shot we can see filming equipment at the right side of the screen.
Continuity mistake: When the six friends arrive at the beach house notice the sand between shots. At first it's all wet due to "flood damage", two seconds later the sand's dry. (00:05:30)
The One with the Thumb - S1-E3
Continuity mistake: When Lizzie holds the tin foil hat up to Phoebe, in one shot she holds it out with one hand, then in the next shot she is holding it in both hands. (00:12:10)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Rachel asks Phoebe if she is a twin, she is wiping the table next to her. In the next shot Rachel has her hands together. (00:11:40)
The One With Joey's Fridge - S6-E19
Continuity mistake: When Joey pushes Ross against the fridge, a box falls from the top of the fridge onto the shelf beside it. A few seconds later when Joey turns to Chandler, we can see over his shoulder that the box is back on the fridge (00:08:55)
The One With the Holiday Armadillo - S7-E10
Continuity mistake: When Ross is on the couch telling Ben about Chanukah, Ben's hair varies from shot to shot. In the first shot it's flat and in the next shot it's spiked up at the front. His hair stays spiked up for a few shots and then returns to flat. (00:06:00)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: During the scene where Phoebe is telling Rachel that Paolo made a pass at her, Rachel unrolls a long length of kitchen roll. During the scene there is a shot over her shoulder where we see that the kitchen roll is as normal and in the next shot we see it unrolled again. (00:15:30)
The One With Princess Consuela - S10-E14
Continuity mistake: When Rachel starts to cry, after Ross brings the champagne, Ross hugs her and holds the glass in his left hand, but when it cuts to the wideshot, the glass is instantly on the table in front of him. (00:10:20)
The One With Phoebe's Dad - S2-E9
Continuity mistake: In the scene where Monica and Ross walk in carrying a few carrier bags, Monica takes out a pack of green plates, but in the next shot she is holding 4 packs of plates - 2 green, 2 red. She did not have time to pick up another pack between shots. (00:08:05)
The One with the Sonogram at the End - S1-E2
Continuity mistake: At the start of the episode when Chandler is talking about Pink Floyd, his legs are both on the table. When the shot changes, his right leg is resting on his left knee. (00:00:50)
The One Where Chandler Takes a Bath - S8-E13
Continuity mistake: When Joey is talking to Chandler about London, Chandler's wedding ring keeps disappearing and reappearing. (00:03:10)
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.