The One With The Embryos - S4-E12
Visible crew/equipment: When Rachel crosses the boys' apartment to see what is in the drawer, if you look up to the right, you can see a glimpse of a boom mic. (00:20:25)
The One With All The Wedding Dresses - S4-E20
Continuity mistake: When Chandler tells Joey that he should go to a sleep clinic, Monica puts her coffee cup on the table, but then it reappears in her hands and disappears repeatedly. (00:02:00)
The One With Chandler In A Box - S4-E8
Visible crew/equipment: When Chandler is telling the friends the meaning of the box, the camera goes a bit too high and all the stage lights and equipment become visible on the ceiling. (00:12:30)
The One With Chandler In A Box - S4-E8
Continuity mistake: In the episode with Chandler in a box, when Rachel is showing Ross all the things she had kept from their relationship in a shoe box, her hair is tucked behind her ear in most shots, but when she says "I keep the gifts that matter", it's all loose, then goes back to how it was in the following shot. Also in the shot before that line she's pointing the horn fossil's tip at Ross, but when it cuts to a front view she's holding the horn's tip and pointing the broken end. (00:16:30)
The One With The Girl From Poughkeepsie - S4-E10
Continuity mistake: In the scene on the train, when the conductor announces Montreal, Ross's head goes from laying on his shoulder to a upright position from one shot to the next. (00:19:00)
The One With The Ballroom Dancing - S4-E4
Continuity mistake: In the Central Perk scene when Joey is telling Monica and Rachel about the dancing, Monica's legs are crossed left over right and then right over left, and back again. (00:10:55)
The One With Rachel's Crush - S4-E13
Continuity mistake: At the very start Ross says to Chandler, "People can hear you," and looks away. The camera cuts to Chandler and Ross is looking at him. (00:00:10)
The One Where They're Going To PARTY! - S4-E9
Revealing mistake: When Monica and Phoebe are in Central Perk, Phoebe is looking through the list of ideas that Monica has come up with for the use of Phoebe's van. If you look closely (most obvious just after Phoebe says "let's plan the wedding reception") you can see that every single page of the notebook Phoebe is holding has the same thing written on it, instead of lots of different ideas that Monica is meant to have had. (00:17:10)
The One With All The Rugby - S4-E15
Factual error: Before Ross goes out to play the second half of the rugby game, Emily gives him some tips on how to hurt the other guys. She singles out one of the guys, saying he doesn't wear a cup. These guys are all English players who love rugby. Rugby players would NEVER wear a cup for rugby is more of a grinding sport than hit and miss. These guys would be crazy to wear a cup.
The One With Joey's Dirty Day - S4-E14
Continuity mistake: In the scene where Mr Heston confronts Joey about using the shower, Mr Heston points at Joey from a seated position. As the shots switch Mr Heston's "pointing" hand also switches. (00:16:30)
The One With The Embryos - S4-E12
Continuity mistake: When Ross takes a bite of the cookies, he says they are not good, and puts it back in the box; in the next wide shot he is seen putting it back in the box again. (00:03:15)
The One Where They're Going To PARTY! - S4-E9
Revealing mistake: When Phoebe is looking at the front page of the "Chelsea Reporter," Monica says her restaurant review is on the back page. However, Phoebe doesn't turn to the back page, but merely flips the paper over, looking at the bottom of the front page (and finding the review there). (00:07:30)
The One With Joey's Dirty Day - S4-E14
Continuity mistake: When Rachel is asking Ross if he will take Emily to the opera, the amount of biscuit in his hand changes throughout. (00:06:15)
The One With The Dirty Girl - S4-E6
Continuity mistake: At the beginning of the episode where they are watching Ross through the window, Phoebe is leaning forward and standing to the left of the pole next to Chandler. When the shot changes to Ross coming through the door, she is suddenly standing up straight and is now behind Joey and Chandler. (00:00:10)
The One With Ross's Wedding, part 1 - S4-E23
Continuity mistake: Rachel is cutting flowers and she and Phoebe are discussing if she loves Ross. Phoebe's necklace switches sides every time the shot goes back to her. (00:07:35)
The One With The Ballroom Dancing - S4-E4
Other mistake: Right at the end of the episode, Phoebe's got a job interview. We see the outside of a massage parlour, but the name on the awning has been superimposed - it moves about a bit while the shot itself is still. (00:20:45)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: In the bar, Kathy is trying to set Chandler up with one of her friends. When the camera is facing Chandler, we see Kathy's hand rubbing Joey's back, but when it cuts to a front shot of Joey and Kathy, her arm is down in front of her. Then without having had enough time to move, her elbow is up on the table and she is holding hands with Joey. (00:16:10)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: In the scene where Monica and Ross discuss how Emily has decided to call off the wedding after Monica suggests they postpone the wedding, Monica is wearing a black watch on her left wrist. This disappears and re-appears a number of times throughout the scene. (00:15:00)
The One With All The Haste - S4-E19
Continuity mistake: Phoebe comes in wearing Santa pants. When Joey and Chandler point this out, Chandler's legs are crossed or uncrossed, depending on the camera angle. (00:04:10)
The One With All The Wedding Dresses - S4-E20
Visible crew/equipment: After Ross announces that he and Emily will be getting married in a month, there's a shot of Joey and Chandler, and when it cuts back to Ross saying, "Yup!" the reflection of the boom mic can be seen on the framed picture hanging on the bathroom door.
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.