The One With Monica's Thunder - S7-E1
Continuity mistake: When Phoebe is singing for the first time, the door behind her is open or closed, depending on the shot. (00:05:00)
The One With Monica's Thunder - S7-E1
Continuity mistake: Chandler is in Rachel's apartment after he has just left Monica; he says to Rachel, "You look great", then Rachel puts her hair behind her ears. The camera cuts to Chandler, then straight back to Rachel, and her hair is not behind her ears. (00:05:05)
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 Monica's Thunder - S7-E1
Continuity mistake: In the scene where Phoebe is telling Joey that his eyes "give him away" he is holding a sandwich; the camera angle cuts to behind him and the sandwich is on the table. (00:15:50)
The One With Monica's Thunder - S7-E1
Continuity mistake: When Monica says "I nearly made you sleep with Ross" her hands are outstretched. The camera cuts and now they are instantly by her side. (00:21:05)
The One With Monica's Thunder - S7-E1
Continuity mistake: The beginning to series 7 takes place right after series 6 ended, basically Chandler has only just proposed to Monica. Yet Monica's hair is visibly different; at the end of series 6 she had no fringe and a center parting, now she has a fringe. Also Chandler, is visibly thinner, with a gaunt face due to losing 20 lbs in between the two series. Rachel also has had a hair cut, her long extensions have been removed and she is back to her usual hair length.
The One With Monica's Thunder - S7-E1
Continuity mistake: When Rachel and Ross go to talk to Monica about why she doesn't want to talk to her, Chandler's tie changes, from on the bed to on his lap.
The One With Rachel's Book - S7-E2
Continuity mistake: Chandler and Monica are having dinner with her parents. He asks Jack how he and Judy got engaged. He starts, "Well I'd gotten Judy pregnant..." Then they talk about how Jack told Monica's friends a "scary story" about how Judy got pregnant because her method of birth control, her diaphragm, had malfunctioned (damaged by a dog). In the rest of the show, however, we are continually hearing that the firstborn, Ross, was "a medical marvel" and that they thought Judy was barren. This apparently is the main reason why they love Ross more than Monica.
The One With Rachel's Book - S7-E2
Continuity mistake: When helmet-wearing Vicar Joey taunts Rachel one too many times she turns the tables on him, so when Joey backs away from Rachel he gets pushed up against the front door, and although he can't move, in the shot facing Joey he's at the left side of the door, but in the side shots he's at the right side of the door.
The One With Phoebe's Cookies - S7-E3
Continuity mistake: While trying out different recipes, at one point Monica's holding a cookie in her hand. It's a whole cookie, but when the angle changes, suddenly a chunk is missing. (00:13:00)
The One With Phoebe's Cookies - S7-E3
Continuity mistake: When Chandler and Jack are placing their robes on the hook, they change positions when Jack looks at Chandler naked. (00:19:39 - 00:20:08)
The One With Rachel's Assistant - S7-E4
Continuity mistake: In the scene when Joey is talking to the producers of Mac and Cheese, he says, "We're not even shooting them anymore?!" As he says this he lifts his left arm up, but in the very next shot his left arm is back down by his side. (00:14:40)
The One With Rachel's Assistant - S7-E4
Continuity mistake: Ross has always said that Carol was his first (and only, until Julie) sexual partner, but in the episode where they tell each others secrets Chandler says Ross slept with the cleaner when they were at college. Some people say that Ross had kept it a secret, and that's why it had never come out before, but considering that he categorically states several times over series one and two how he'd only EVER been with Carol, getting quite upset at times, that explanation just doesn't ring true.
The One With The Engagement Picture - S7-E5
Continuity mistake: When 'Hums While He Pees' hands Phoebe the tickets for the ballroom dancing, they switch hands depending on the camera angle. She takes them in her right hand, but in the shot from behind she has them in her left hand. (00:08:25)
The One With The Nap Partners - S7-E6
Continuity mistake: In the Maid Of Honour sequence, Joey's head starts to move one way. The camera cuts and it's no longer moving. (00:14:00)
The One With The Nap Partners - S7-E6
Continuity mistake: Monica and Chandler are in their apartment talking about Monica's worries that Chandler would dump her if she put on weight. When we first see Monica, the bite mark in her apple is quite small, but a few seconds later, it is suddenly much bigger without her eating any more of it. (00:18:00)
The One With The Nap Partners - S7-E6
Continuity mistake: When Ross and Joey wake from their nap on the couch while everyone else is watching them, Marvin the Martian stuffed doll is lying on the floor between the couch and wall, but in the closeups Marvin the Martian is on the couch.
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)
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 With Ross's Library Book - S7-E7
Continuity mistake: When Chandler says "lie" his hand is by his side, then up. (00:07:20)
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.