The One With Monica and Chandler's Wedding, Part 2 - S7-E24
Continuity mistake: During an earlier episode Monica gave up her dream wedding dress so Chandler could have The Swing Kings at their wedding. So it is a little surprising that the band they actually have are called Jungle Swing.
The One Where They're Up All Night - S7-E12
Factual error: In the opening scene, Ross remarks on the stars the group is gazing at. In reality, there would be far too much light pollution in this part of New York City to be able to see any celestial bodies such as comets or stars.
The One With The Flashback - S3-E6
Revealing mistake: When Monica offers Joey lemonade and he takes off his clothes and is behind the lamp, the tops of his boxer shorts are visible when he says "Were you just gonna give me lemonade?" Broadcasts may crop off this bottom part, DVD doesn't. You can also see his gray boxer shorts through the plant in the previous shot.
The One With Rachel's Sister - S6-E13
Continuity mistake: When Jill drops her shopping bags on the coffee shop table, the small blue Tiffany's bag is visible on the end of the 'line'. But it is not visible in any shots after Rachel comes in. Additionally, when Rachel grabs the bags and leaves, Ross seems to pick the little blue bag up off the floor, though it is not shown how the bag got there. (00:05:35)
The One with the East German Laundry Detergent - S1-E5
Visible crew/equipment: When Rachel jumps into the cart, and the rude woman is bending over, you can see a mike pack underneath the back of her T-shirt. (00:21:30)
The One With the Baby on the Bus - S2-E6
Factual error: In the one where Joey and Chandler lose baby Ben on a transit city bus, they later have to pick him up at the transit city headquarters. The man working there assumes that one of them is the father. There is no way that a city official would just give away a baby without checking for ID or a birth certificate or proof of fathership of the child. (00:14:30)
The One After Joey And Rachel Kiss - S10-E1
Revealing mistake: Just as they are getting ready to leave Barbados, Ross goes to speak to Chandler in the hotel room. The view outside the window is obviously a painting, or hanging. (00:10:55)
The One with the Thumb - S1-E3
Continuity mistake: When Joey comes into Monica and Rachel's apartment and sits down, he puts his towel on his lap. In the next shot it is over his shoulders, then back on his lap again. (00:18:30)
The One Where Underdog Gets Away - S1-E9
Continuity mistake: When Rachel gets her mail off Monica she puts it down on the couch and picks up a sealed envelope. The camera cuts and the envelope is now open with no time to open it. (00:07:25)
The One With the Male Nanny - S9-E6
Continuity mistake: The hallway outside Ross' new apartment (series 6 onwards) always changes. Sometimes, the apartment is at the end of a dead-end corridor, and there is a plant outside, covering up the wall; sometimes you can go past the apartment and turn left or right; or, as in this episode, you can only turn right immediately after you have passed the apartment. (00:26:00)
The One Without The Ski Trip - S3-E17
Continuity mistake: Just before Ross leaves the rest stop, Phoebe goes over to him and says,"Maybe we can see a movie or something. Or the rodeo." When she says this, she takes a step back from Ross twice. (00:20:20)
The One With All The Kissing - S5-E2
Continuity mistake: In the scene after Monica pretends to drop socks in the hallway in order to prevent Rachel from telling Ross she still loves him, Rachel goes back into the apartment and locks Monica out. Monica then starts to pound on the door with her bare left hand. The scene changes to the apartment and Ross opens the door to let Monica in. All of a sudden Monica is pounding the door with her right hand wrapped in a sock. (This continuity problem is for the widescreen version, where we see a closeup of Monica not seen on the DVD, in the hallway with a bare hand right before Ross opens the door. However, on the DVD there are extra shots of Ross and Rachel after seeing Monica's bare hand pounding and then seeing her with the sock on her hand, which gives her ample time to put it on). (00:11:45)
The One With The Princess Leia Fantasy - S3-E1
Continuity mistake: When Joey says "we'll call that plan B, all right?", Monica has her chin in her hand. When it cuts to a different angle, she's got both arms on the table. Rachel also goes from having both hands clasped under her chin to having one arm resting on the table and one reaching for coffee. (00:05:10)
The One With Monica's Thunder - S7-E1
Continuity mistake: When Joey is asking how old he looks, his hands are in his pockets. In the next shot he is gesturing, then instantly his hands are back in his pockets. (00:09:00)
The One With Rachel's Date - S8-E5
Continuity mistake: When Ross asks Rachel how the date went, she grabs the bag she is holding. Next shot she is touching her coat. (00:18:20)
The One with the Blackout - S1-E7
Continuity mistake: At the end of the episode, as Chandler turns to the security camera, he is stood right next to the door. In the next shot he has jumped about 2 feet away from the door instantly. (00:21:20)
The One with George Stephanopoulos - S1-E4
Continuity mistake: When Joey jumps over the sofa to give Ross the tickets, one of the pages of Ross's magazine is sticking out towards him. In the next shot it is gone. (00:02:40)
The One In Vegas, Part 1 - S5-E23
Visible crew/equipment: When Ross is leaving Rachel's, a stagelight can be seen by the bathroom. (00:12:00)
The One With Joey's Bag - S5-E13
Visible crew/equipment: When Joey walks into Monica's apartment the first time, a stage light can be seen by the window near the bathroom. (00:10:00)
The One With Ross's Library Book - S7-E7
Continuity mistake: At the very beginning, Joey is eating some cereal and then says he has to go to work, so puts it down on the table - but in the rest of the scene it vanishes. (00:00:15)
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.