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 Where Rachel is Late - S8-E22
Visible crew/equipment: After Rachel has told Ross it's 100° outside and she starts walking towards the door, a camera is visible to the far right (Widescreen version only). (00:09:30)
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.
The One With Joey's Dirty Day - S4-E14
Continuity mistake: After Chandler gets back from the strip club, when he throws his jacket and scarf they both land on the rug between the recliner and coffee table, but when Chandler sits in his recliner the jacket is gone and the scarf is lying on the floor on the opposite side of the table.
The One With All The Rugby - S4-E15
Continuity mistake: When Rachel comes home and excitedly tells Monica that she felt Joshua's pulse, there are two handbags hanging from Rachel's shoulder which vanish between shots, but then they reappear on her shoulder.
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: Before Monica stomps on Paul's watch, the chair closest to the fridge does not have fabric backing, then when Ross eats the Oreo and walks toward his jacket the chair closest to the fridge is now fabric backed and his jacket hangs from a chair with no fabric, plus there are only four chairs around the table. However, when Ross picks up his jacket this chair is now fabric backed, the chair closest to the fridge doesn't have a fabric back yet again, and there are now five chairs around the table. (00:27:00)
The One Where They're Going To PARTY! - S4-E9
Continuity mistake: When Joanna stops Rachel from leaving work Rachel's holding her handbag and jacket, and there's still one bag hanging on the wall hook, but when Joanna mentions the "drinking binge" there are two bags hanging from the hook, then it's back to just one bag, yet when Joanna calls Sophie there's nothing on the hook and Rachel has both bags.
The One With The Dirty Girl - S4-E6
Continuity mistake: When Ross and Cheryl are standing in the hall outside Cheryl's apartment, she's holding her keys in her right hand when she asks Ross if he'd like to come in, but after Ross makes the joke about Homo erectus, Cheryl opens her hand and the keys have vanished from her hand.
The One With The Ballroom Dancing - S4-E4
Continuity mistake: While Treeger chews out Rachel for not properly disposing the pizza box in the trash chute, the room's door is closed, then wide open, and then closed again.
The One With The Jellyfish - S4-E1
Other mistake: When Chandler and Monica are sitting at the table after playing Gin, Chandler's bottle of Yoo-hoo is on the table, but the label reads "Yooh" because the brand name has been partially covered - the blue letters "oo" are yellow to match the background.
The One With The Apothecary Table - S6-E11
Continuity mistake: While Monica and Chandler confront Janine in Joey's apartment, we can see the drawing of the fire breathing dragon and the ape on the Empire State building on the Magna Doodle, then when Joey tells Chandler and Monica to leave because he needs to talk to Janine, the drawing differs - note the dragon's scales, its position, the building, etc.
The One With Rachel's Inadvertent Kiss - S5-E17
Continuity mistake: While the two couples are at the restaurant, Chandler has a beer and Gary, Monica, and Phoebe have margaritas, and the amount of liquid in Gary's margarita glass keeps changing, back and forth from less to more.
The One With The Ride-Along - S5-E20
Continuity mistake: When Rachel is at Ross' apartment she gets the blender and while she's standing at the window talking to Monica on the phone the kitchen door is closed, then it's wide open, and then closed again.
The One With The Girl Who Hits Joey - S5-E15
Continuity mistake: When Phoebe gives the housewarming gifts to Ross and also when Ross opens the door for the neighbor, his apartment number is "3B" and his neighbor's door is "3A," but in episode 14, "The One Where Everybody Finds Out," the door can be seen three times during the episode, and the number is "201" each time.
The One With The Girl Who Hits Joey - S5-E15
Continuity mistake: When Ross tells Chandler and Rachel that he's running out to buy name tags for his party, in the next shot the water bottle beside Rachel does not have its cap on, but it cuts to Rachel's closeup and the cap is on. Other differences can be seen as well, such as Rachel's hair and an extra utensil.
The One With Princess Consuela - S10-E14
Continuity mistake: After Ross announces his tenure, when Rachel starts crying and tells him that she got fired and didn't get the other job, her right hand is up by her face, but in the next shot her right hand is holding the glass of champagne and it's her left hand up by her face.
The One Where Everybody Finds Out - S5-E14
Continuity mistake: When Phoebe pours the white wine Chandler empties his glass to the last drop, but in following shots Chandler's glass has wine again, which then keeps vanishing and reappearing.
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.