The One With Ross's Inappropriate Song - S9-E7
Continuity mistake: When Ross is holding Emma, the position he is holding her changes.
The One With The Joke - S6-E12
Continuity mistake: In the coffee house scene when Joey reveals that he is working there, the strap on Joey's apron goes back and forth from being twisted around his neck and then straightened out. (00:10:50 - 00:12:50)
The One With Ross's Library Book - S7-E7
Continuity mistake: When Monica says, "she's not going to like that chicken either," Chandler's arms are crossed. Then the shot changes, and they're now by his face. (00:07:50)
The One Where Rachel Finds Out - S1-E24
Continuity mistake: When Rachel and Monica are discussing whether or not Rachel should meet Ross at the airport she's taking a bottle of red wine out of the refrigerator, but when she places it on the table in the next camera shot the bottle of wine is suddenly a bottle of white wine. (00:16:25)
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: While everyone is booing Mike and Phoebe, the guy sitting behind Phoebe changes position in every shot.
The One With Christmas in Tulsa - S9-E10
Continuity mistake: When Wendy is coming onto Chandler her hands are at the top of his tie. When the shot changes, her hands are halfway down his tie.
The One Where Chandler Crosses The Line - S4-E7
Continuity mistake: In the final scene wherein Phoebe finally agrees to play guitar again, Ross moves from the armchair to the sofa. As he sits down, his arm is by his side, but when the camera angle shifts, it is on the back of the sofa. (00:20:50)
The One Where Ross Can't Flirt - S5-E19
Continuity mistake: In the scene where the gang is watching Joey's "substitution" video, you can easily see the reflection on the television that no one is sitting on the couch, but when the camera goes back to the gang, they're sitting down on the couch. (00:19:30)
The One With The Worst Best Man Ever - S4-E22
Continuity mistake: In the scene where the veterinarian gives the ring back to Ross, there is a dog in the background that goes from standing up to lying down to standing again depending on the shot.
The One With The Apothecary Table - S6-E11
Continuity mistake: When Phoebe and Rachel are first talking about the apothecary table, there are two piles of CDs on it. In the side shot you can see that the two CDs on the top of the pile in front of Rachel are overhanging by a few centimetres. A few shots later, however, the top two CDs are overhanging on a different side. (00:06:05)
The One Where Joey Dates Rachel - S8-E12
Continuity mistake: When Phoebe is playing Ms. Pacman to erase Chandler's high scores, the door of the apartment can be seen open. When she loses and starts swearing, Ross enters the apartment with Ben, opening the door. (00:19:10)
Continuity mistake: The bathroom in Chandler and Joey's apartment constantly changes in the direction it opens in. Sometimes the doorknob is on the left, sometimes it is on the right. The changes occur throughout the seasons, and in some cases the changes occur in back-to-back episodes.
The One With Phoebe's Wedding - S10-E12
Continuity mistake: After Ross greets Phoebe at the rehearsal dinner, he moves toward Mike as if he is going to speak to him. The camera cuts to Phoebe and Ross is now standing behind her, facing away from where Mike is.
The One With The Apothecary Table - S6-E11
Continuity mistake: When Phoebe and Ross are talking about Ross buying his sheets at the flea market, Phoebe says, "Ross, come on, you gotta loosen the purse strings a little" with her hands by her side. The camera cuts and her hands are now out in front of her.
The One With The Princess Leia Fantasy - S3-E1
Continuity mistake: When Ross gets takes out a Popsicle and says who dated a stripper, he takes a bite of his Popsicle and and the camera cuts to Rachel, then back to Ross and the bite has vanished.
Continuity mistake: The white dog that Joey gives Chandler sometimes has red lips and sometimes does not.
The One With the Male Nanny - S9-E6
Continuity mistake: When Ross says, "Not knowing when to shut up, that's my thing," the amount of pizza he has eaten changes between shots. (00:21:50)
The One With Phoebe's Birthday Dinner - S9-E5
Continuity mistake: Near the start, when Chandler asks if anyone else smokes, a woman with a brown jacket is seen getting a pack of cigarettes out her pocket. The camera cuts and now she has one in her mouth and is lighting it. (00:02:35)
The One Where Rachel is Late - S8-E22
Continuity mistake: At the end, when Rachel and Ross leave for the hospital, Rachel grabs the keys. In the close-up, when Rachel asks, "Do you think we have time?" she holds the keys up in her left hand while Ross holds her empty right hand. It then cuts to a medium shot, and the keys are now in Rachel's right hand, the same hand Ross holds. (00:20:40)
The One Where Rachel Tells... - S8-E3
Continuity mistake: At the start, before Monica and Chandler leave for their honeymoon, note the chairs around the kitchen table. When Rachel walks in to tell the couple to have a good honeymoon, the chair in front of the fridge and the chair in front of the stove have switched places.
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.