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 Racecar Bed - S3-E7
Visible crew/equipment: While Rachel and Ross are at the apartment, Rachel tries to convince Ross to have brunch with her father so they can get along, and in the shots facing Rachel we can see the movement of a crew member in the window's reflection.
The One With Frank Jr. - S3-E5
Visible crew/equipment: After Joey nearly drills a hole into Chandler he heads over to Monica's to get a beer, and just as Joey walks into the apartment the camera can be seen at the left side of the screen.
The One With The Metaphorical Tunnel - S3-E4
Visible crew/equipment: After Chandler gives Janice the contact paper for his dresser drawer, when Chandler stands up and says, "Yes, I did," we can see the camera at the right side of the screen.
The One With Rachel's Dream - S9-E19
Visible crew/equipment: After Monica and Phoebe apologize to each other at Javu, when it cuts to Rachel walking into the apartment we can see part of the hole in the wall beside the bulletin board, and the light fixture above it is gone.
The One With Rachel's Other Sister - S9-E8
Continuity mistake: When Rachel tells Amy that she'll let her use the Ralph Lauren discount Amy hugs her, and the glass Amy's holding in her left hand switches to her right hand and then back to her left hand, between shots.
The One With Rachel's Other Sister - S9-E8
Continuity mistake: When Ross, Rachel, and Amy arrive at Monica and Chandler's apartment the table is set, and there are folded napkins in napkin rings on the plates at each place setting, but after Amy insults Joey all the napkins have vanished.
The One With the Male Nanny - S9-E6
Continuity mistake: When Mike and Phoebe are at Central Perk, Mike pulls his apartment key out of his pocket and he's holding the head of the key with the shank side up, but next shot he's holding the shank with the head side up.
The One With the Sharks - S9-E4
Continuity mistake: After Monica, Rachel, and Joey have been discussing Joey's problem Monica asks him if he thinks sharks are sexy, and when Joey asks about the Little Mermaid it cuts to Monica's closeup and the water bottle cap is on, but in the next closeup of Rachel the cap is off.
The One Where No One Proposes - S9-E1
Revealing mistake: While everyone's looking at Emma through the window, when Jack makes the comment about the first grandchild, someone in the background walks by and causes the hallway's set wall to wobble behind them.
The One With The Cooking Class - S8-E21
Visible crew/equipment: When Joey and Monica are leaving the cooking class, the teacher asks about the difference between hollandaise and béarnaise sauce, and in the next shot of Monica and Joey we can see the shadow of the moving boom mic at the top left side of the screen.
The One With Chandler's Work Laugh - S5-E12
Continuity mistake: During Ross and Janice's conversation in Central Perk, when Ross comments, "You're ending this with me because I'm too whiny?" both his and Janice's left arms are up, but it cuts to the next shot and it's their right arms which are up, then it happens again when Ross says, "I've become so whiny that I annoy you?! Janice!" they both have their right arms up, but next shot it's their left arms up.
The One Where Joey Tells Rachel - S8-E16
Continuity mistake: At the end of the previous episode, Joey tells Ross that he has feelings for Rachel just as Ross is about to walk out of Central Perk, and there's a vase of tulips on the table beside the door and two things hanging on the coat hooks. The opening scene of this episode continues at the same moment of Joey's admission, and the vase has vanished and there's only a sweater hanging on the coat hooks.
The One Where Joey Tells Rachel - S8-E16
Continuity mistake: When Monica and Chandler get back to the apartment Chandler tosses his coat over the back of the couch, and while they're talking about soulmates and a house of cheese we can see Chandler's coat in the reflection on the TV, but then it vanishes in the reflection, though it reappears when Chandler says that he wants to take Monica out to a restaurant.
The One Where Rachel Tells... - S8-E3
Continuity mistake: When Rachel sits down in Ross' apartment, she places her magazine down on the table with its binder facing the audience, but while Ross goes on about their one night just being fun the binder is now facing the opposite way on the table, and then it's flipped back to the original position.
Continuity mistake: When Ross introduces himself to Mona, the reception place card that Mona's holding switches from her left hand to her right hand, between shots.
The One with All the Poker - S1-E18
Continuity mistake: After the game at Ross' apartment Chandler takes Phoebe's cursed money, and when Ross wants to settle up with "the big Green poker machine who owes fifteen," Rachel's sitting down at the table in front of Ross, but then it cuts to Rachel who's suddenly standing up near the table. (00:12:30)
The One with All the Poker - S1-E18
Visible crew/equipment: After Rachel gets the call that she did not get the job, when she places the five dollar bet and asks if anyone has a problem with that, in the next wideshot as everyone says, "NO!" the reflection of moving equipment can be seen in the window behind Monica.
The One Where Heckles Dies - S2-E3
Visible crew/equipment: When Mr. Heckles is being taken away, just as the officer starts to follow the gurney the shadow of the moving boom mic can be seen on the wall behind him, at the right side of the screen. (Widescreen version).
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.