Visible crew/equipment: Most of the time when they show the Central Perk logo on the window. If you look inside you see the set because the wall doesn't go far enough.
The One Where Monica Gets a Roommate - S1-E1
Visible crew/equipment: We get a close-up of the coffee pot when Rachel asks if anyone would like some. In this shot, there are 3 stage lights reflected clear as day. (00:27:50)
The One with the Sonogram at the End - S1-E2
Visible crew/equipment: While Rachel's looking for the ring she says, "Now I'm gonna have to return the ring, without the ring, which makes it so much harder," and at the start of the next shot the reflection of a screen can be seen moving on the window behind Monica. (00:05:55)
The One with the Sonogram at the End - S1-E2
Visible crew/equipment: When Ross announces that Carol's pregnant, after Chandler says, "Kind of puts that whole pillow thing in perspective..." in the next shot of Ross and Monica we can see a crew member's sleeve at the left side of the screen. (00:07:30)
The One with the Sonogram at the End - S1-E2
Visible crew/equipment: Ross stands up to tell his parents about what has happened with Carol. When he does he walks round the table and stands under a very visible boom microphone. (00:12:05)
The One with the Thumb - S1-E3
Visible crew/equipment: At the end, just as Monica walks off a tape marker can be seen on the floor where she was stood. This is as the credits are on screen. (00:22:30)
The One with George Stephanopoulos - S1-E4
Visible crew/equipment: At the end, while Rachel's on the phone talking about the Visa card, in the shots facing Monica, Phoebe, Joey, and Chandler when they're playing Twister, the end of the set wall can be seen at the right side of the screen. (00:15:10)
The One with George Stephanopoulos - S1-E4
Visible crew/equipment: When Monica's looking out the window at George Stephanopoulos with the binoculars, the end of the set wall and overhead studio lighting can be seen at the left side of the screen. (00:15:20)
The One with George Stephanopoulos - S1-E4
Visible crew/equipment: When the guy delivers the wrong pizza, in the shot after Monica grabs the pizza box to hand to Phoebe and shouts that they have George's pizza, the tape marks can be seen on the floor with one of them between the delivery guy's feet.
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 East German Laundry Detergent - S1-E5
Visible crew/equipment: When the horrible woman enters, the boom mike is visible in the mirror at the top of the screen. (00:10:10)
The One with the East German Laundry Detergent - S1-E5
Visible crew/equipment: When the horrible woman enters, a tape marker is visible on the floor by the machines. (00:11:20)
The One with the East German Laundry Detergent - S1-E5
Visible crew/equipment: While Chandler is trying to break up with Janice, and Phoebe comes to help, a cameraman is visible in the background (outside of Central Perk) walking from right to left. The camera cuts away from the window angle, then cuts back, showing another cameraman walking from left to right. (00:19:15)
The One with the East German Laundry Detergent - S1-E5
Visible crew/equipment: When Rachel jumps into the cart, and the rude woman is bending over, you can see a mike pack underneath the back of her T-shirt. (00:21:30)
The One Where Nana Dies Twice - S1-E8
Visible crew/equipment: When Monica and Ross meet up with their parents and their nana, a boom mic is visible at the top of the screen. (00:04:20)
The One Where Nana Dies Twice - S1-E8
Visible crew/equipment: When the scene cuts from Joey and Mr. Geller watching the game, to several people watching the game, a boom mic is visible at the top of the screen and the camera tilts slightly. (00:19:00)
The One Where Underdog Gets Away - S1-E9
Visible crew/equipment: When Monica shuts the TV off while watching skiing, there is a closeup of the screen as it goes blank. You can see reflected in the blank screen an image of a man in a long sleeved shirt and tie, who does not correspond to any of the characters in the room. (00:08:00)
The One Where Underdog Gets Away - S1-E9
Visible crew/equipment: When Joey walks into Central Perk after the VD poster montage, a tape marker is visible on the floor behind Chandlers spot on the sofa. (00:10:40)
The One Where Underdog Gets Away - S1-E9
Visible crew/equipment: When Chandler runs into the apartment to say Underdog has gotten away, a crew member can be seen reflected in the sun shaped mirror behind him walking through the back of the shot. It can't have been Rachel, Monica, Phoebe or Joey because Joey is sitting down and Rachel, Monica and Phoebe are nowhere near the mirror. (00:14:05)
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.
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.