The One With Rachel's Phone Number - S9-E9
Visible crew/equipment: After Chandler has climbed down the fire escape and shows up in the hallway where Joey's been waiting with a baseball bat, when they both walk into the apartment the opening in the wall can be seen beside the door. We can see the opening again after Joey finds Chandler's suitcase.
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 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.
The One With Ross' New Girlfriend - S2-E1
Deliberate mistake: The door to Monica's bedroom is open in many shots, such as when Ross introduces Julie, and we can see that there's a wall (with a framed picture) directly opposite the doorway, but when Phoebe's giving Monica the haircut they're in Monica's bedroom and there's no such wall in front of the door.
The One with the Ick Factor - S1-E22
Visible crew/equipment: After Ethan and Monica talk about "at gunpoint" being a place, when Monica asks Ethan to stay over it cuts to a shot facing Ethan and we can see that the back wall is less than half a wall.
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 With Ross' Step Forward - S8-E11
Continuity mistake: It's obviously December, with the set decorations and Rachel's reference to sitting on multiple Santas' laps because she is randy from her pregnancy hormones. The characters establish that it's Rachel's fourth month of pregnancy...but she became pregnant about six weeks before Monica and Chandler's May 15 wedding. Her fourth month would have been in July, not December.
The One with the Thumb - S1-E3
Continuity mistake: When Monica is first explaining to her friend in the restaurant that her friends don't like anyone she dates, she is putting food onto a plate. The food changes between shots.
The One With All The Wedding Dresses - S4-E20
Visible crew/equipment: When Ross asks Rachel how she feels about him marrying Emily, several stage lights can be seen in the reflection of the TV during the entire scene.
The One Where Chandler Can't Cry - S6-E14
Continuity mistake: In this episode Chandler says he can't cry, but in series 4 when he broke up with Kathy he was seen crying, and other times - it's just this episode it's apparently an issue.
The One With Monica's Thunder - S7-E1
Continuity mistake: When Monica says "I nearly made you sleep with Ross" her hands are outstretched. The camera cuts and now they are instantly by her side. (00:21:05)
The One With The Triplets - S5-E3
Factual error: Joey always grabs his back when he gets his pain from his kidney stones. This is a classic referral pattern for pain when the stones are in the kidney. However, the doctor says that the kidney stones were too close to the bladder. Stones in the ureters classically refers the pain to the groin area. (00:02:40)
Continuity mistake: In the beginning in the coffee house, Ross is explaining to Phoebe and Rachel what 'Unagi' is. The blonde lady behind Phoebe is wearing light blue jeans. She then puts a brown leather jacket on, however in the next shot it has gone, only to reappear and disappear again. (00:03:20)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: During the scene where Chandler is playing hide and seek with the duck and the chick, the duck moves from facing the opposite way to him, facing the same way, and then the opposite way as Joey walks in. (00:13:35)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: When Rachel says 'everything looks like jammies now' she is holding up a t shirt. In the next shot as she throws it down, it is now folded up. (00:17:00)
The One with the East German Laundry Detergent - S1-E5
Continuity mistake: When Rachel is holding the ice pack to Ross's head at the end of the episode, Ross' left hand is lowered, then in the next shot, it is raised to his head. (00:21:35)
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)
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.