The One With The Free Porn - S4-E17
Continuity mistake: Near the end, when Emily walks into the house, the TV should be on with the porn. But instead, it's turned off.
The One With The Blind Dates - S9-E14
Continuity mistake: When Ross and Rachel are at the coffee shop after their dates, Rachel's necklace disappears and reappears twice. (00:20:20)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Ross shows Chandler and Joey the can of "Carol's favorite beer" it's unopened as Ross makes a crack about how he "should have known," but in the next shot we see Ross lifting the can and drinking from it, even though he never actually pulled open the stay-tab between the two shots (viewed in widescreen). [In the extended version of the episode you see Ross open the can before taking a drink. In the standard version, the mistake is valid]. (00:09:55)
The One After the Superbowl, part 2 - S2-E13
Continuity mistake: When Chandler removes his clothes while he and Susie are in the bathroom, Susie picks up both of his shoes and holds them in her arms in all of her closeups, but when Chandler's standing on the toilet one of his shoes is still on the floor in the wideshots.
The One With The Joke - S6-E12
Continuity mistake: In the coffee shop Phoebe is not in the scene. After Joey puts his apron on, the camera shows Phoebe is next to Rachel. Monica asks Joey if he works there, he says no, a customer calls "waiter" and in a full view of the coffee shop Phoebe is no longer there. (00:09:45 - 00:10:55)
The One Where Ross Got High - S6-E9
Continuity mistake: When Ross asks Rachel to step into the hallway, we see the whipped cream atop the trifle dessert plates on the table, but while Joey gives everyone acting lessons all the whipped cream is gone, then the dollops of whipped cream reappear once again.
The One With The Proposal, part 1 - S6-E24
Continuity mistake: When the waitress opens the door to the kitchen to tell Monica about a complimentary customer, in the first shot the waitress pulls the door open and stands in front of it, but in the next shot she's pushing the swing door into the kitchen poking her head in to talk to Monica. (00:20:35)
The One With The Prom Video - S2-E14
Continuity mistake: When Chandler throws the basketball and breaks the lamp, the base falls onto the chair with the shade, but when Joey walks into his room the lamp base is back up on the desk.
The One With The Truth About London - S7-E16
Continuity mistake: In the flashback scene where Chandler and Monica are in the hotel room, there are three paintings on the wall of flowers, etc. In the original scene from ‘the one with Ross' wedding part 2' the pictures are completely different.
The One With The Halloween Party - S8-E6
Continuity mistake: While Phoebe is calling out Ursula for lying to Eric, her hair continuously changes position from behind her shoulder to in front of her shoulder depending on the camera angle. (00:14:50)
The One With The Lesbian Wedding - S2-E11
Continuity mistake: 12 hours before the wedding, Monica asks Joey to speed it up. Here she is stirring a bowl. In the next few seconds you can see Monica holding some condiments in her hand. After Phoebe answers, Monica is stirring again and the condiments are nowhere to be seen.
The One With the Pediatrician - S9-E3
Continuity mistake: After Rachel's 3 AM phone call to her pediatrician, when Rachel suggests using Dr. Gettleman she raises her left hand and points at Ross, but in the next closeup it's her right arm that's raised.
The One With The Blind Dates - S9-E14
Continuity mistake: When Phoebe and Joey are at the coffee house discussing their plan to ruin Ross' blind date just before Joey says, "Ooh I hear that's bad" the pendant on Phoebe's necklace is smooth and blue. When the camera pans to Phoebe after Joey says his line the pendant is flipped over and reveals a new design. This continues to happen throughout the scene. (00:07:20)
The One With The Cheap Wedding Dress - S7-E17
Continuity mistake: Chandler's chair appears in this episode again, although it was broken in 7/13 and thrown out. It is not in the room in 7/14-16, but in 7/17 it's there again, then it is gone again in 7/18.
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: After Joey runs out of the apartment to go make the fake videotape, when it cuts to Rachel walking back into the apartment the table with the tablecloth is quite far away from the front door, but when Caitlin knocks and Ross rushes to go flirt with her that table is near the door. (00:13:35 - 00:15:15)
The One The Morning After - S3-E16
Continuity mistake: When Monica and Phoebe get excited about being a "Waxine girl", Monica puts the mop upright against the counter and the mop falls over, but in Phoebe's closeup we see the mop is still upright behind her. (00:06:20)
The One The Morning After - S3-E16
Continuity mistake: While Rachel and Ross are fighting, when Rachel realises that she was at Ross' apartment at the same time as his "little friend", Ross holds out the newspaper (so she can hit him) with his left hand, but in the next shot it's held out in his right hand. (00:13:40)
The One With The Fake Party - S4-E16
Continuity mistake: After Rachel spins the bottle and it points to Joshua, when she crawls over to him on her hands and knees the bottle's in front of her knees, but in the next shot facing Joshua the bottle is to her side, near her feet.
The One With The Fake Party - S4-E16
Continuity mistake: When Joey takes the steaks out of the freezer, Phoebe's holding a napkin up by her face with her left hand while her right hand is down and empty, but in the next shot it's her right hand that's up with a napkin.
The One With The Fake Party - S4-E16
Continuity mistake: When Emily says that she'll go first at spinning the bottle we see the door behind Ross is wide open, but after Emily spins the bottle that door is closed, yet it's wide open again during the game and then closed.
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.