The One with the East German Laundry Detergent - S1-E5
Visible crew/equipment: While Chandler is trying to break up with Janice, and Phoebe comes to help, a cameraman is visible in the background (outside of Central Perk) walking from right to left. The camera cuts away from the window angle, then cuts back, showing another cameraman walking from left to right. (00:19:15)
The One with the Thumb - S1-E3
Continuity mistake: When we first see Chandler in his office, his sleeve is rolled below his elbow. In the next shot, it is above his elbow. (00:12:30)
The One with the Evil Orthodontist - S1-E20
Other mistake: This is the episode where a guy across from Monica's apartment is spying on them. Joey says he caught him spying into his and Chandler's apartment yesterday. This is impossible considering the layout of the building i.e. the fact that the only part of Joey and Chandler's apartment which faces toward the spying guy has the stairwell and hallway blocking it from view (not to mention the protruding part of Monica's apartment). There is another window in Joey's apartment but it faces completely the wrong direction. The bathroom has no windows. The angles just don't add up. (00:00:40)
The One With The Lottery - S9-E18
Continuity mistake: In TOW the lottery tickets series 9 just after Chandler has got the job and he sits down you can see Joey's hand in the right side of the screen however when the shot changes Joey moves his hands up to the position we have just seen. The shot changes back and his hand is in the same position. (00:18:50)
The One With The Fertility Test - S9-E21
Visible crew/equipment: When Monica and Chandler are on the couch in the fertility clinic after Monica says "Nobody's gonna wanna watch that" the camera switches to a wider shot. When it does this the boom mic swiftly moves out of view. (00:12:20)
The One with the Blackout - S1-E7
Continuity mistake: Rachel takes the cat to apartment number 8 and Mr. Heckles opens the door. However, in this episode, the girls' apartment is labeled number 5 which means that number 8 is above them. In later episodes, Mr. Heckles always says that the noisy girls upstairs are keeping him from doing things. [It makes a bit more sense later on in the series when Monica and Rachel's appartment changes to number 20, which is when Mr Heckles starts to complain about the noise (although in the flash back episode when Phoebe is still living with Monica it seems apparent that they actually know Mr Heckles, which Phoebe shows no sign of when she and Rachel are looking for the owner of the cat).] (00:13:40)
Continuity mistake: The number on Pheobe's cab changes throughout series 2 and 3. 7E85 and 2X85 respectively.
The One With Princess Consuela - S10-E14
Continuity mistake: When Phoebe sits down in the coffee house and tells Chandler and Monica her new name, she takes her jacket off twice. (00:07:20)
The One with the East German Laundry Detergent - S1-E5
Character mistake: In the laundromat, there is a sign where the word clothes is misspelt cloths. (00:16:00)
The One With The Truth About London - S7-E16
Audio problem: During Rachel's scene sitting on the couch with Ben talking about her and Ross and her sisters, Rachel's mouth moves in any scene showing Ben, but without her being heard.
The One With The Video Tape - S8-E4
Audio problem: When Joey says "this is not fair to her, let's just forget the tape", his mouth stops moving after he says "forget", making it look like the audio is coming from nowhere. (00:12:07)
The One With All The Jealousy - S3-E12
Visible crew/equipment: After the barbershop quartet sing their "Ross song" to Rachel at her office, at the start of the next shot the camera can be seen at the right side of the screen, just as Ross walks out of Rachel's bedroom.
The One Where Chandler Can't Remember Which Sister - S3-E11
Visible crew/equipment: When Phoebe and everyone else hear the guy upstairs with the "pogo stick" Joey gets up to give him the "butt kicking of a lifetime," and just as Joey walks around the couch we can see a crewmember and camera, at the left side of the screen.
The One With The Rumor - S8-E9
Continuity mistake: When Ross hands Rachel the Yams in order to break the tension, the amount of food on both Ross' and Will's plates changes between shots. Also several wine glasses appear on the table. (00:12:30)
The One With The Fake Party - S4-E16
Continuity mistake: After Rachel explains her plan for the fake party, just as Emily opens the apartment door the message written on the Magna Doodle reads "Joey enjoys the," but when Emily and Ross are standing in front of the door the message reads "Joey eats little."
The One With The Fake Party - S4-E16
Visible crew/equipment: While Rachel's explaining how to play 'spin the bottle' the bottle she spins points to Gunther, and in the shot facing Gunther we can see the end of the set wall, at the right side of the screen.
The One With The Fake Party - S4-E16
Visible crew/equipment: After Ross finds out that Monica told Emily about how he used to play dress up and host make-believe tea parties, when Monica stands up we can see set lighting near the bathroom, at the top of the screen.
The One With All The Rugby - S4-E15
Visible crew/equipment: When Phoebe walks into the apartment while Ross and Joey are watching rugby on TV, the boom mic and a bit of the overhead studio lighting can be seen right before she sits down, and the boom mic can be seen again when Joey stands up after Chandler walks in with Janice.
The One With The Embryos - S4-E12
Continuity mistake: At the end, when Monica asks about the garbage bags the apartment's front door is closed, but when Rachel opens the drawer and freaks out about what's inside, the front door is wide open (for the camera) as she kicks the drawer shut, and then the door is closed again. (Widescreen).
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: While Joey and Chandler are at the apartment with Ross, a watch suddenly appears on Joey's wrist when he announces that he and Chandler are done with the bookcase, then the watch vanishes and reappears again in the following shots at the apartment. (00:15:40)
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.