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 All The Wedding Dresses - S4-E20
Visible crew/equipment: After Ross announces that he and Emily will be getting married in a month, there's a shot of Joey and Chandler, and when it cuts back to Ross saying, "Yup!" the reflection of the boom mic can be seen on the framed picture hanging on the bathroom door.
The One With The 'Cuffs - S4-E3
Continuity mistake: The handcuffs that Joanna uses on Chandler have a single metal bar connected via pivot to a double bar with the lock area. When Joanna first cuffs Chandler, the double bars on both sides face up at his wrists, but when Chandler's on the phone and when Rachel unlocks his right cuff, the handcuffs are going round his wrist the opposite way with the single bars facing up - quite impossible. Then the same thing occurs when Rachel handcuffs Chandler to the filing cabinet.
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: Just as Phoebe walks into the apartment with the duck and chicken (to help find Monica's hidden presents), a crew member's leg/foot can be seen at the bottom left side of the screen.
The One Where Ross Hugs Rachel - S6-E2
Other mistake: When Monica's trying to evoke some kind of an emotional reaction from Rachel, who's polishing her toenails, there's a bottle of Cutex nail polish remover on the table with part of the brand name painted black, so it reads "Cu Ex" on the label.
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 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 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)
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.