The One With Joey's Porsche - S6-E5
Visible crew/equipment: When the camera follows Phoebe as she is going to clean up the vase she knocked over, the foot of the baby is still visible in the shot and you can see how someone picked it up as the foot goes straight up. Then as the camera moves back revealing there are only 2 babies, you can see a shadow of a crew member moving on the ground. (00:16:00)
The One With The Worst Best Man Ever - S4-E22
Continuity mistake: When Joey says that Ross needs to choose his best man before handing out of the ring, the camera cuts to Chandler. In the background you can see Joey getting up from the chair, but when it cuts back to Joey he is sitting down and then gets up in that shot. (00:02:01)
The One with the Birth - S1-E23
Character mistake: Outside the room in the corridor where Carol is waiting to deliver her baby, the poster outside giving directions says "Nusery", not "Nursery". (00:02:15)
The One Where Rachel Tells... - S8-E3
Continuity mistake: In a deleted scene, the security guard opens Monica's case. There is a blue item on top, when the camera changes, this item has disappeared.
The One With Rachel's Crush - S4-E13
Continuity mistake: When Monica waves the smell of cookies in the guys' new apartment, you see Ross right in front of Phoebe when they walk over, then you see Ross in front of Rachel as they go in.
The One With The Hypnosis Tape - S3-E18
Continuity mistake: When Pete is trying to ask Monica out at the diner she works at, she rejects him while placing the bell jar over the cheesecake and the pie server on the counter. In the next shot, the bell jar is nowhere to be seen and the pie server is up on the cake stand with less cheesecake. Then the bell jar reappears over the cheesecake. (00:07:00)
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)
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.