The One With The Memorial Service - S9-E17
Continuity mistake: In the beginning when Joey tries to spin the ball with his finger, it falls, hits the table and bounces to the background. In the very next shot it is seen falling behind Chandler on the couch.
The One In Vegas, Part 2 - S5-E24
Continuity mistake: When Monica and Chandler kiss under the table, as "Everybody Loves Somebody" plays, Monica is shown smiling. She then closes her mouth. In the next shot, she is smiling again without any time to do so.
The One With The Football - S3-E9
Character mistake: Ross tells Chandler to run a "post pattern to the left." (And Chandler quickly agrees.) In football, this is a type of passing play where Ross would hold on to the ball while Chandler runs straight down the field immediately after the snap, eventually veering diagonally to the left once he's a ways down field to receive a pass from Ross. The play that they actually run is completely different. It's a running play known as a sweep. (A toss sweep to be precise.) There is no possible way that anyone either making play calls or running them would ever confuse a post vs. a sweep. (00:10:10)
The One With All The Resolutions - S5-E11
Continuity mistake: Immediately when the opening scene starts, we hear from the crowd on the television that there is less than 60 seconds to midnight. However, 60 seconds later in the scene, the countdown still hasn't reached zero.
The One With Mac & C.H.E.E.S.E. - S6-E20
Continuity mistake: As Chandler is writing on the board to remind himself about Joey's audition, the door opens as he's writing "Joey", and there are a bunch of squiggly lines under the y. As Chandler is thinking back at that moment and it moves to present tense, he looks over to the board, and the squiggly lines are different.
The One Where The Stripper Cries - S10-E11
Continuity mistake: When the stripper first walks in, he is wearing a uniform with a circular badge. A few moments later the badge is not circular, it is a shield type and shape and the uniform appears to be different.
The One With The Ball - S5-E21
Continuity mistake: When Rachel is outside the coffee shop trying to sell the cat, in between shots, it goes from a standing to sitting position multiple times.
The One Where They're Up All Night - S7-E12
Visible crew/equipment: Right after Tag wonders if he can see his apartment from there, when Ross says "wow, look at all the stars" a boom-mic is visible for a short amount of time.
The One with the Boobies - S1-E13
Continuity mistake: When they're all sitting together and having pizza, talking about parents, Rachel has no napkin. When she says: "Just think, in a couple of years we get to turn into them", a bunched up napkin appears beside her on the couch. After Chandler's remark it has disappeared again, then it appears and reappears again between shots. (00:21:10)
The One Where Emma Cries - S9-E2
Continuity mistake: When Chandler wakes up and spits out the balloons, his legs change between shots from being crossed to having one foot on the table and back, at least twice. (02:00:00)
The One with the Ick Factor - S1-E22
Revealing mistake: In the scene where they are in the coffee shop talking about Rachel's dream about Chandler. Rachel is carrying a coffee pot around then sits down. When she sits she puts her hand under the coffee pot, which would be extremely hot if it were real.
The One With Mac & C.H.E.E.S.E. - S6-E20
Visible crew/equipment: A stage light can be seen at the top left hand side of the screen just after Joey slams the door shut after practising a scene with Phoebe. It is visible in front of the blue picture on the wall.
The One With The Ultimate Fighting Champion - S3-E24
Continuity mistake: Ross buys the Ultimate Fighting Combo, a large pop and popcorn at the fight, but when he and Monica sit down, they are never seen again. Furthermore, when Monica talks to Pete at the ring, you can see that the Combo was not set on the floor.
The One With The Princess Leia Fantasy - S3-E1
Continuity mistake: In the seen where Monica is shouting "why hasn't he called" she is holding a juice bottle, but when the shot cuts to Phoebe she is holding a glass.
The One with the Dozen Lasagnas - S1-E12
Visible crew/equipment: When Joey and Chandler walk into their apartment and the table falls over when Joey puts the key on it, the edge of the set and studio lighting are visible at the top of shot. The end of the set wall is also visible at the right side of shot. (00:04:50)
The One With The Cooking Class - S8-E21
Revealing mistake: Just as the scene cuts from the streets to the baby store, the camera wobbles slightly.
The One With Ross's Grant - S10-E6
Continuity mistake: When Phoebe tells Rachel and Monica that Mike lets her keep her box of hair, Rachel's chopsticks are down, by the plate. In the next shot, she's holding them close to her mouth.
The One With Ross's Library Book - S7-E7
Visible crew/equipment: When Chandler puts a plate down next to the Chinese food, and Monica walks in a boom mic can be seen over her head. (best on DVD). (00:06:20)
The One With The Soap Opera Party - S9-E20
Visible crew/equipment: When the roof party begins, above the door a boom mic is visible.
The One With The Birthing Video - S8-E15
Continuity mistake: When Chandler tells Rachel and Monica to look at the screen when the baby has come out, his arm goes from pointing at the TV to on his lap.
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.