The One With Ross's Library Book - S7-E7
Continuity mistake: When Rachel asks Ross what's wrong, her hand is up. Shot changes and it's down.
The One With Ross's Library Book - S7-E7
Continuity mistake: When Janice sees Monica's ring, she's wiping her hands with a rag, then it disappears. (00:03:50)
The One Where Paul's The Man - S6-E22
Visible crew/equipment: In the scene where Rachel is trying to hide Ross from Paul, when he's about to go into Paul's bedroom after he says "Did you really hear a bear?", look at the window behind Rachel when the shot is on her. You can see a large light reflected in the glass and it moves slightly (not just in conjunction with the camera, it moves before the camera shifts).
Visible crew/equipment: When Ross and Rachel realise they are in an empty apartment, studio lighting can be seen over their heads in the ceiling as Ross leads Rachel into the centre of the room. (00:04:35)
The One Where Chandler Doesn't Like Dogs - S7-E8
Visible crew/equipment: When Chandler sees the empty dog basket on the floor at Ross' there is a panning shot to the door. If you look carefully, you can see a crewmember wearing blue jeans and white shoes. (00:17:25)
The One With The Cake - S10-E4
Continuity mistake: When Jack and Judy leave Joey's apartment, there is a birthday banner on the counter that has fallen off. When Monica, Chandler, Joey and Phoebe are racing the toys to see who gets to go, the banner is back on. In this time no-one has fixed the banner as there is not enough time for them to do so off camera and the people on camera don't fix it. It's most clear that it's back on when Monica is saying, "Technically, our dog never crossed the finish line". (00:14:05 - 00:17:05)
The One with the Monkey - S1-E10
Visible crew/equipment: After Max tells Phoebe that he and David got the grant and are leaving for Minsk on January 1st, when Phoebe goes to see David, just as she knocks on the door a boom mic can be seen moving at the top of the screen, before it cuts to David's closeup. (00:12:15)
The One Without The Ski Trip - S3-E17
Continuity mistake: When Phoebe is saying how she gets out of tickets, she is pointing at herself in one shot and not in the very next shot. (00:05:50)
The One With The Football - S3-E9
Continuity mistake: At the end, Chandler points at Joey, and when he does so, his sleeve goes from rolled down to rolled up instantly. (00:22:10)
The One With The Football - S3-E9
Visible crew/equipment: Just before Monica tackles Chandler to the floor, there is a shot with a very large boom mic visible at the top of the screen (only visible in fullscreen version). (00:13:10)
The One With The Football - S3-E9
Continuity mistake: Chandler says he's been preparing for something his whole life. When he says this, his right sleeve is rolled up. Camera changes, and now it's rolled down. (00:10:40)
The One With The Football - S3-E9
Continuity mistake: When Joey tells Chandler to prepare to feel very bad about himself, he has the football in his right hand. When the shot changes, it's in his left. (00:10:30)
The One With The Football - S3-E9
Continuity mistake: When Joey introduces himself to the Dutch lady, the way and where he holds the football changes between successive shots. (00:08:40)
The One With The Football - S3-E9
Continuity mistake: When Rachel asks Ross if not catching Joey was part of his football strategy, she has her hands outstretched in one shot and then on Ross' shoulder in the next. (00:07:10)
The One With The Football - S3-E9
Visible crew/equipment: When Ross is explaining the playing area near the start, the boom mic once again dips into view and follows him (only visible in fullscreen version). (00:05:40)
The One With The Football - S3-E9
Visible crew/equipment: Pay close attention when we see the gang outside for the first time. In the upper left-hand corner, you can see a boom mic moving about in the shot, clear as day. (00:04:30)
The One With Rachel's Big Kiss - S7-E20
Other mistake: Matthew Perry supposedly really likes wearing a tux that turns out to have been made for Diane Keaton. Except that Diane Keaton is 5'7" and Matthew Perry is 6 feet tall, and also considerably larger than the very slim Diane. He wouldn't even be able to put it on.
The One With The Free Porn - S4-E17
Factual error: We know that Phoebe is three months pregnant, because it's the day after the party where she told Joey that she was three months along. But the ultrasound shows three little lumps that in no way resemble a three-month fetus. An actual three-month fetus is the size of a jellybean.
The One With Phoebe's Cookies - S7-E3
Character mistake: When Phoebe brings in her frozen cookie and Monica tastes it to try to determine the ingredients, she says "I definitely taste nutmeg." She then goes on to brag about how that's the difference between a professional and a layman. Except that later when she finds the actual recipe, it's from the Nestle Toll House bag - and that contains no nutmeg.
The One With The Ball - S5-E21
Factual error: There is no way that a cop and a masseuse could have afforded an apartment in Brooklyn Heights, even in the '90s. (Brooklyn Heights was unaffordable in the '80s.) "Rent control" doesn't explain it either, as a vacant apartment - which is what they are looking for - would be raised to "market rate" if a rent-stabilized tenant moved out. This can't be written off as a character mistake: Gary is not naive or uninformed, and his accent makes clear that he is a New Yorker.
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.