The One With Ross's Tan - S10-E3
Continuity mistake: Ross' tan keeps changing shades. It gets lighter then darker, then lighter again. (00:12:40)
The One With Ross's Tan - S10-E3
Visible crew/equipment: When Rachel is talking to Monica about how weird the thing with Joey was, behind her in a yellow vase, you can see a huge light blur reflected from a stage light. (00:12:50)
The One With Ross's Tan - S10-E3
Continuity mistake: When Ross gets a spray on tan, there is one shot where he is being sprayed in the face. Instant camera cut then it's on his chest, not near his face. (00:19:35)
The One With Ross's Tan - S10-E3
Continuity mistake: Chandler walks away from Ross's apartment at the end. But he walks to where the kitchen should be. (00:21:15)
The One With Ross's Tan - S10-E3
Continuity mistake: In the scene where Joey and Rachel are sitting on the couch in their apartment after they decide their relationship is hopeless Rachel puts her head on Joey's shoulder. Her hair is on her face but when she lifts her head up in the very next shot her hair is now behind her right ear which she couldn't have done because her hands never left her lap. (00:22:00)
The One With Ross's Tan - S10-E3
Visible crew/equipment: When Monica walks into the apartment and announces to Chandler that they're not friends with Phoebe anymore, the hole in the wall beside the door (which is usually hidden behind the picture) can be seen when she walks in and also when Phoebe and Amanda walk in.
The One With Ross's Tan - S10-E3
Visible crew/equipment: After Ross goes in for his disastrous tan, when it cuts to Joey and Rachel walking into their apartment the hole (used for a camera angle) in the wall can be seen at the right side of the screen. As an aside, when Ross leaves Monica and Chandler's apartment later just as Rachel walks in, that hole is seen in the hallway and it's covered up.
The One With Ross's Tan - S10-E3
Visible crew/equipment: After Monica gets the call from Chandler about the fake car accident and leaves Phoebe with Amanda, in the next shot of Joey walking into his apartment the bulletin board that usually hangs beside the door is cut to about a quarter of its size, and it's covering part of the hole in the wall.
The One With Ross's Tan - S10-E3
Other mistake: Chandler asks Monica "are you trying to do a British accent?" after she does an impression of Amanda. In the next shot, you see Chandler mouthing the line again.
The One With The Cake - S10-E4
Visible crew/equipment: When Monica is talking to Chandler about waking Emma up, as she walks over to him, crew lights are visible in the top left corner of the screen. (00:05:35)
The One With The Cake - S10-E4
Audio problem: When Chandler says, "Ironically, that was the Maxim thing", you can tell it's an overdub. The volume and the background noise is different, and we don't see a shot of him speaking these words. (00:11:20)
The One With The Cake - S10-E4
Continuity mistake: When Monica's mum and dad are leaving the party, they are standing next to each other while her dad explains why he can not drive anymore. Her mum has both hands in front of her holding on to her purse, in the next shot one arm is around Monica. (00:13:50)
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 Cake - S10-E4
Continuity mistake: Phoebe's robot in the toy race is behind Joey's. Joey then says unlucky and picks his toy up. Phoebe then picks hers up and it is about a metre in front of Joey's. The toy can't move that fast. (00:16:40)
The One With The Cake - S10-E4
Revealing mistake: When Rachel and Ross are on the way back, if you look at Ross you can see a purple tinge to the side of his face. This is from projecting the background. (00:17:30)
The One With The Cake - S10-E4
Continuity mistake: When Ross and Rachel are driving back, Ross starts to put the seatbelt on. Instant cut, and he has his seatbelt on. (00:17:35)
The One With The Cake - S10-E4
Continuity mistake: Just before Rachel and Ross come back from the cake store, Monica is holding Emma on her lap. Emma is holding a green toy in one shot, when the camera angle changes it is nowhere to be seen. (00:19:30)
The One With The Cake - S10-E4
Continuity mistake: Near the end, Monica rearranges Emma's stuffed toys in order of size. When you next see them they are jumbled up, then later they return to being neatly lined up again. (00:19:40)
The One With The Cake - S10-E4
Continuity mistake: At the end after Rachel and Ross have come back from the cake store, when they are asking Emma how old she is, we can see Joey walking to sit down, then in the next shot he is sat down, then walking to sit down again. (00:20:25)
The One With The Cake - S10-E4
Visible crew/equipment: After Joey describes Emma's funny Elmo cup to Phoebe, it cuts to Monica and Chandler walking in, and to the right of the door we can see the bulletin board is cut in size partially covering the hole in the wall. Later, when Phoebe and Joey leave after the wind-up toy race not only is this visible again, but through the hole we can see the other hole in Monica and Chandler's wall.
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.