The One In Vegas, Part 1 - S5-E23
Visible crew/equipment: After Phoebe tells Chandler that Monica wants to make up and that he needs to find her, when Chandler leaves the room we can see the end of the set wall, with the hallway and another hotel room door, at the left side of the screen.
The One In Vegas, Part 2 - S5-E24
Visible crew/equipment: At the start of the Netflix version, when the recap scenes from 5x23 Part 1 are being shown, in the shot of Phoebe, Monica, and Chandler on the airplane the end of the cabin set can be seen beside Chandler, at the right side of the screen.
The One In Vegas, Part 1 - S5-E23
Visible crew/equipment: While Phoebe, Monica, and Chandler are with gladiator Joey in the casino, Joey notices the tension between Chandler and Monica, and when he asks what's going on a tape mark can be seen on the floor at the feet of the extra standing at the slot machine, then more tape marks can be seen at the same spot after Monica sarcastically wishes Chandler happy anniversary.
The One With Chandler's Work Laugh - S5-E12
Visible crew/equipment: After Joey and Phoebe place their betting cash on the counter, when Joey says, "Let's get the contestants out of their isolation booths," the end of the set wall can be seen behind the foosball table. (Widescreen version).
The One In Vegas, Part 1 - S5-E23
Visible crew/equipment: When Phoebe, Monica, and Chandler walk into Caesars Palace Casino, Phoebe notices Joey, and in the next wideshot we can see the camera panning around (it's on a camera crane, presumably filming the overhead angle seen in this scene), at the top right corner of the screen. (Widescreen version).
The One With All The Resolutions - S5-E11
Visible crew/equipment: Monica and Chandler leave the apartment to go "do laundry," then Phoebe heads out to the airport. When Joey takes a seat, the reflection of camera movement is visible on the window, above the sink. Then Rachel and Joey tell each other they can't share what they each know, and just as Rachel sits down beside Joey, once again we can see the reflection of camera movement above the sink. (00:19:30 - 00:20:20)
Visible crew/equipment: When Ross notices that Ugly Naked Guy is back, in the next exterior shot we can see the top of the set as everyone gathers at the window.
The One With All The Resolutions - S5-E11
Visible crew/equipment: After his fiasco with the leather pants, powder, and lotion, when Ross is at Monica's and tells the others he's going to kiss Ben goodnight, as he starts walking to the bedroom the camera can be seen at the right side of the screen.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: While Joey's looking for the "hot girl" the second time, after he knocks on the little girl's door and asks if her mother, sister, or babysitter are a hot girl, he runs off when she shouts for her father, and when Joey stops in another hallway full of doors the shadow of the moving boom mic can be seen on the door beside him.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: When Rachel gets back from her first interview, just as Chandler says, "Oh hey," we can see the boom mic at the top of the screen.
The One With Rachel's Inadvertent Kiss - S5-E17
Visible crew/equipment: When Joey is standing at Monica's window waving to Ross, the end of the set wall with movement on the soundstage can be seen at the left side of the screen.
The One Where Everybody Finds Out - S5-E14
Visible crew/equipment: When Ross is using the binoculars to watch Ugly Naked Guy's apartment, the end of the set wall can be seen behind him.
The One Where Everybody Finds Out - S5-E14
Visible crew/equipment: When everyone's looking out Monica's window into Ugly Naked Guy's apartment, and see all the gift baskets he's been sent, the end of the set wall in Monica's apartment can be seen.
The One Where Everybody Finds Out - S5-E14
Visible crew/equipment: While Phoebe and Rachel are at Ugly Naked Guy's apartment, when Phoebe looks out the window and sees Monica and Chandler the end of the set wall in Monica's apartment can be seen.
The One With The Inappropriate Sister - S5-E10
Visible crew/equipment: When the passerby drops his lit cigarette in Phoebe's charity bucket, the end of the department store facade can be seen at the left side of the screen, and later when Phoebe hands off the bell to her replacement the end of the right side facade can be seen as well.
Visible crew/equipment: While everyone's in the apartment Ross is on the balcony talking to Emily on the phone, and in the mirror we can see the reflections of Phoebe and someone else wearing a blue shirt (with a large white pattern) and blue pants (looks like jeans), but none of the others are actually dressed like that.
Visible crew/equipment: Just as Monica and Rachel walk into the storage room, at the start of the shot the camera casts a shadow on the head sculpture which moves as the camera moves.
The One With The Inappropriate Sister - S5-E10
Visible crew/equipment: At the start as Joey comes out, watch the tv and you will see a line of stage lights. (00:00:05)
The One With All The Thanksgivings - S5-E8
Visible crew/equipment: Just as Monica finishes her story and Chandler is shown, the reflection of the stage lights is visible in the TV. (00:18:20)
The One With All The Thanksgivings - S5-E8
Visible crew/equipment: Monica is explaining her Thanksgiving to Chandler; when she drops the knife there is a shot looking up from the floor. A crew member's shadow is visible on the ceiling. (00:07:10)
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.