The One With The Birth Mother - S10-E9
Visible crew/equipment: When Joey's telling Phoebe about having been at a nice restaurant with his date and how she took fries off his plate, in the shots facing Joey in the restaurant we can see the end of the set wall, with a 2x4 framed plywood board and a heavy duty extension cord hanging there.
Other mistake: When Phoebe and Ross are at JFK, some of the people who are at that airport are the same people who are also at Newark. It's particularly funny to see the people behind Ross and Phoebe at JFK when Phoebe claims they're on their honeymoon, and when it cuts to Rachel at Newark as she's looking for her boarding pass, those same people are there too.
The One With Ross's Tan - S10-E3
Visible crew/equipment: When Monica walks into the apartment and announces to Chandler that they're not friends with Phoebe anymore, the hole in the wall beside the door (which is usually hidden behind the picture) can be seen when she walks in and also when Phoebe and Amanda walk in.
The One With Ross's Tan - S10-E3
Visible crew/equipment: After Ross goes in for his disastrous tan, when it cuts to Joey and Rachel walking into their apartment the hole (used for a camera angle) in the wall can be seen at the right side of the screen. As an aside, when Ross leaves Monica and Chandler's apartment later just as Rachel walks in, that hole is seen in the hallway and it's covered up.
The One Where Joey Speaks French - S10-E13
Visible crew/equipment: After Rachel tells the others that her father had a heart attack and Ross takes two of Monica's cookies, when it cuts to the shot of Central Perk it's from a low angle, and the studio lighting rigs at the ceiling can be seen through the windows.
The One With Phoebe's Wedding - S10-E12
Continuity mistake: When Monica and Phoebe are talking about having a harpist versus Marjorie playing the steel drums, the handbag strap over Monica's shoulder vanishes and reappears.
The One After Joey And Rachel Kiss - S10-E1
Visible crew/equipment: After Chandler starts removing Monica's braids from the shower curtain rings, when it cuts to Joey and Rachel walking into their apartment, we can see the hole in the wall at the right side of the screen. And it's seen again when Ross walks in on Rachel and Joey kissing.
The One With The Cake - S10-E4
Visible crew/equipment: After Joey describes Emma's funny Elmo cup to Phoebe, it cuts to Monica and Chandler walking in, and to the right of the door we can see the bulletin board is cut in size partially covering the hole in the wall. Later, when Phoebe and Joey leave after the wind-up toy race not only is this visible again, but through the hole we can see the other hole in Monica and Chandler's wall.
The One With Ross's Tan - S10-E3
Visible crew/equipment: After Monica gets the call from Chandler about the fake car accident and leaves Phoebe with Amanda, in the next shot of Joey walking into his apartment the bulletin board that usually hangs beside the door is cut to about a quarter of its size, and it's covering part of the hole in the wall.
The One Where The Stripper Cries - S10-E11
Continuity mistake: Towards the end of the episode, Ross and Chandler come into Monica's apartment and Ross asks where Rachel is. Watch the objects in Monica's hands, in the shot from behind she puts them on the table, and then does it again in the shot from the front. (00:24:00)
The One With Princess Consuela - S10-E14
Continuity mistake: When Phoebe sits down in the coffee house and tells Chandler and Monica her new name, she takes her jacket off twice. (00:07:20)
Continuity mistake: When Phoebe's driving Ross to the airport in her cab the rear view mirror vanishes from the windshield, then it reappears and disappears again. Additionally, when they start their drive the cab doesn't have a side view mirror on the passenger side, but en route the cab has acquired one, then it vanishes again.
The One After Joey And Rachel Kiss - S10-E1
Visible crew/equipment: Ross, Joey, and Chandler are chatting in the plane. After the camera zooms out when Chandler heads to sit next to Monica, a cameraman holding a camera is visible on the left of the scene. [HD/widescreen version]. (00:13:25)
Other mistake: The photo in Rachel's passport at the airport is not of her. (00:29:45)
The One After Joey And Rachel Kiss - S10-E1
Visible crew/equipment: After Monica whips her hair around and smacks herself in the face with all the shells in her hair, it cuts to Ross at his apartment, and the end of the set wall can be seen at the right side of the screen, with the sound stage visible. (00:21:45)
The One Where Estelle Dies - S10-E15
Continuity mistake: Near the end when Chandler is talking to Janice at one point his hands go from pointing straight ahead to his pockets. (00:17:20)
The One With Rachel's Going Away Party - S10-E16
Continuity mistake: The level of beer in Ross' bottle goes up and down between shots as he's talking to Joey at the kitchen table. (00:12:00)
The One With The Cake - S10-E4
Visible crew/equipment: When Monica is talking to Chandler about waking Emma up, as she walks over to him, crew lights are visible in the top left corner of the screen. (00:05:35)
The One Where Estelle Dies - S10-E15
Revealing mistake: When Chandler and Monica are at the door of the house you can see that it is a fake background behind Chandler. (00:09:35)
The One Where Estelle Dies - S10-E15
Continuity mistake: The second time Ross visits Mr. Zelner, he hands him a pterodactyl egg replica. The hole in the egg switches between the top or bottom, depending on the shot. (00:14:40)
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.