The One With The Jellyfish - S4-E1
Continuity mistake: When Monica gets stung by the jellyfish they are standing on a part of the beach that is wet and close to the surf but when she is saying "damn the jellyfish" the part they are standing on is dry and further from the water.
The One Where They All Turn Thirty - S7-E14
Continuity mistake: In The One Where They All Turn Thirty, it is Rachel's birthday. They show flashbacks of all the other Friends' thirtieth birthdays despite Monica's birthday supposedly being months after Rachel's. Plus, earlier on in the show Joey was 25 when Monica was 26, and since Monica and Rachel were in the same grade Joey should actually be a year younger than them, not a year older.
Continuity mistake: When Rachel and Monica knock on Danny's door and he opens it, there is a wall a few feet behind him. Later, when Rachel shows up at Danny's apartment to cancel their date, she enters into an open room and there is no longer a wall in front of the door. Danny's apartment is supposed to be one floor directly below Chandler and Joey's apartment. This was obviously the same set for their apartment that was used for Danny's and the wall was added for that shot to disguise it.
The One With The Inappropriate Sister - S5-E10
Continuity mistake: When Rachel and Monica are arguing about her meddling, Monica raises her hand to her face and you see a sparkly engagement ring on her finger. Her character wasn't engaged but Courteney Cox was at the time. (00:07:05)
The One With The Ultimate Fighting Champion - S3-E24
Continuity mistake: Joey's hair is suddenly much shorter after the intro with Robin Williams and Billy Crystal - everyone's wearing the same clothes so no time is meant to have passed.
The One With Rachel's Dream - S9-E19
Continuity mistake: When Rachel is telling Monica about her dream at Central Perk, Phoebe enters and leaves her bag on the couch but it falls. However, in the next shot, her bag is right at her side on the couch. (00:14:40)
The One With The Proposal, part 2 - S6-E25
Continuity mistake: Right after Monica says, "You wanted it to be a surprise", the camera pans to Joey who shuts the door firmly, however in the next shot you can see the door gliding back open and then shutting again. (00:18:40)
The One Where Rachel Quits - S3-E10
Continuity mistake: In the classroom Ross has his head down because he didn't want to be called on about the number of boxes he sold. The mother brown bird walks behind him and her right hand is on her hip while she is clearing her throat. In the next shot her right arm is pointing upward.
The One With The Giant Poking Device - S3-E8
Revealing mistake: After Monica runs into the bedroom screaming "I wrecked your child" Rachel says "I hope it's funny when you're in hell." When Rachel is saying that line you can see the shadow of Monica on the wall in the room indicating she is just standing there waiting to make her entrance again.
The One With The Jellyfish - S4-E1
Continuity mistake: In the last few seconds of season 3, Ross closes the door immediately after he steps through it. However, in the opening seconds of season 4, we see this same event from the other side of the door. This time however, Ross doesn't close the door for at least 15-20 seconds after entering the room.
The One With The Cake - S10-E4
Audio problem: When Chandler says, "Ironically, that was the Maxim thing", you can tell it's an overdub. The volume and the background noise is different, and we don't see a shot of him speaking these words. (00:11:20)
The One With The Joke - S6-E12
Continuity mistake: In the scene in the coffee shop where Joey tells his friends about him closing down the coffee shop when he went to the audition, as Rachel says "He left work in the middle of the day to do a personal errand?" two men walk past behind the group. As she continues talking just a few moments later, the same exact men walk past again.
The One With The Apothecary Table - S6-E11
Visible crew/equipment: Right after the "I'll Be There For You" intro, the camera shows the view from outside the "Central Perk" café. As it does so, a couple of stage lights are visible through the upper part of the window, obviously part of the set on the inside of the building.
The One Where Ross Meets Elizabeth's Dad - S6-E21
Continuity mistake: When Chandler is about to put the toilet paper on the table, the picture with the little girl is facing Phoebe. In the next shot, the picture is facing Chandler. Two shots later, it's facing Phoebe again, and then Chandler again in the next shot. (00:08:15)
The One With The Free Porn - S4-E17
Continuity mistake: In the beginning when Rachel says "Do we really have to watch this while we eat." From the front view Rachel's arms are crossed and then from the back their are by her side and back with no time to turn.
The One With The Tiny T-Shirt - S3-E19
Continuity mistake: When Ross is looking through the peephole, his right foot moves inward with no time to move.
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 Ross Is Fine - S10-E2
Continuity mistake: When Owen is telling his parents Chandler paid him $50 not to tell them he knows he is adopted, Chandler says "which technically now you should give back" and he is looking at Owen. In the next shot his back is to Owen.
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: When Ross is looking at Emma in the coffee house he turns and looks at the guys on the couch and mouths the word "what." Then in the next shot he does it again.
The One With Joey's Interview - S8-E19
Continuity mistake: In the opening scene, Joey is holding the Soap Opera Digest magazine. In one shot it is open but folded, then in another it is open all the way, then it's folded again, then it's open again, then it's closed, and then it's folded again. The change is never shown.
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.