The One Where Ross Got High - S6-E9
Plot hole: Monica is trusting Rachel to make a traditional English trifle and being a top chef (or an even averagely intelligent human being) she would know what ingredients are used in one. So there's no way she could have missed Rachel cooking beef with peas and onions during the cooking process. Rachel's cooking it right there in Monica's kitchen while everyone else is there!
Other mistake: When Joey announces that he has a job in the new Al Pacino movie, Ross is holding a bottle of peach Snapple with part of its brand name whited out, so it reads "S pple" on the label. (00:13:50)
The One With Ross' New Girlfriend - S2-E1
Continuity mistake: When Chandler has just got back from seeing Joey's 'bad' tailor, Joey says, "That's how they do pants". Chandler turns to look at him twice. (00:17:30)
The One With The Giant Poking Device - S3-E8
Revealing mistake: When Monica and Rachel are banging their heads on the wood, it is clear that they aren't hitting them hard enough to result in the winces that are apparent. (00:09:00)
The One With The Princess Leia Fantasy - S3-E1
Plot hole: In the beginning the Friends enter Central Perk and see that there are some other people on their couch. Monica looks fine and happy - even though later in the episode the other Friends say that she's been depressed for some days now, and she doesn't leave her apartment.
The One With All The Resolutions - S5-E11
Character mistake: In one of the scenes where Phoebe is teaching Joey guitar, she asks him to demonstrate the "Iceberg" chord. To refresh his memory, Joey refers to the chord as "G Sharp." In reality, the only way to play a G Sharp chord is with a barre chord, and as such the fingering is not exclusive to it.
The One Where Everybody Finds Out - S5-E14
Continuity mistake: In the scene where Chandler turns on music when he and Phoebe are going to "have all the sex" you see Phoebe's arm begin to move in the right corner of the screen. When it cuts to her she has not yet begun dancing, and when he goes to turn the music on again it is a duplicate clip from the initial shot.
The One With the Pediatrician - S9-E3
Continuity mistake: In the scene where Phoebe asks Mike to prove that he plays piano, his arm goes from resting on the couch to on his lap from one shot to the next. After Phoebe tells him "You are really good," his left arm is resting on the couch. But in the next shot when Phoebe continues talking, his arm is resting on his lap.
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: In the episode before, Rachel's hair is just underneath her shoulders. Then, a few weeks later when Ross gets back from Spring Break, it's a lot longer than before.
The One Where Rachel Tells... - S8-E3
Visible crew/equipment: When Monica is yelling at the other couple on their honeymoon, the things on the shelves behind them shows the reflections of set lights. (00:15:25)
The One Where Old Yeller Dies - S2-E20
Continuity mistake: A woman and a man in a blue shirt exit the coffee shop twice.
The One With The Bullies - S2-E21
Continuity mistake: A woman and a man in a blue shirt exit the coffee shop twice.
The One With The Bullies - S2-E21
Visible crew/equipment: While driving the cab, Phoebe is wearing a blue shirt. When the car stops suddenly, a crew member wearing a white shirt is visible driving the car.
The One With The Halloween Party - S8-E6
Audio problem: When Rachel greets Mona at the door of the apartment, visually she says "Hi" but the audio is " Hi Mona. "
The One With Chandler's Dad - S7-E22
Character mistake: When Chandler says "and there's daddy", an extra behind him chuckles in time with the studio audience. He's the only one laughing and it happens right on cue with the audience.
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: When Rachel tells Ross that she "kind of lost" Marcel it cuts to an exterior shot from the balcony, and if we look past Rachel and Ross we see there's a table against the wall, under the Maïna La Voyante poster, but that table does not exist in previous or following shots. (00:10:15)
The One With Joey's Bag - S5-E13
Continuity mistake: When Phoebe realises she just met her dad she turns to her friends to share the news, and in the closeup she's holding a pair of 3D glasses in her right hand and Monica's holding a pair in her left hand, but in the next wideshot both of those glasses have vanished. (00:09:15)
The One With Joey's Bag - S5-E13
Continuity mistake: When Chandler speeds up the timer for his turn he places the timer down at the right side of the pillow, but it's at the opposite side of the pillow when he lies down. Then before Monica starts giving Chandler his massage, she sets the timer and places it near the foot of the bed which we see in Chandler's closeup, but in the next shot when she says, "Say goodbye to sore muscles," oblivious to Chandler's pain, the timer is lying by the pillows behind her, near the head of the bed. Note, in the "uncut scene" version, Chandler grabs the timer to speed up his torture, and Monica takes it out of his hand thinking he was going to add more time, then she places the timer behind her. (00:00:25)
The One With The Apothecary Table - S6-E11
Audio problem: When Phoebe puts her feet up on Ross' apothecary table, Ross asks her not to put her feet up on his "new...old sheet." During the pause right after he says "new," there's a cut to a different angle and you can see his mouth form the words "old sheet." Then it cuts to a different angle and we actually see and hear Ross say "old sheet." (00:12:05)
The One With Rachel's Going Away Party - S10-E16
Revealing mistake: The twins are dolls. They don't move, and the way they are being carried indicates they don't weigh much.
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.