The One With Phoebe's Wedding - S10-E12
Continuity mistake: When Monica and Phoebe are talking about having a harpist versus Marjorie playing the steel drums, the handbag strap over Monica's shoulder vanishes and reappears.
The One Where Rachel's Sister Babysits - S10-E5
Continuity mistake: Rachel and her sister are in Rachel's room. When Rachel picks up Emma, Emma has nothing in her arms, but in the next shot when Rachel is holding her, she has a toy in her arms though she had no time to get one.
The One Where The Stripper Cries - S10-E11
Continuity mistake: Towards the end of the episode, Ross and Chandler come into Monica's apartment and Ross asks where Rachel is. Watch the objects in Monica's hands, in the shot from behind she puts them on the table, and then does it again in the shot from the front. (00:24:00)
The One With Princess Consuela - S10-E14
Continuity mistake: When Phoebe sits down in the coffee house and tells Chandler and Monica her new name, she takes her jacket off twice. (00:07:20)
Continuity mistake: When Phoebe's driving Ross to the airport in her cab the rear view mirror vanishes from the windshield, then it reappears and disappears again. Additionally, when they start their drive the cab doesn't have a side view mirror on the passenger side, but en route the cab has acquired one, then it vanishes again.
The One With The Home Study - S10-E7
Continuity mistake: When Ross and Rachel are pushing Emma on the swing, the legs of Emma's pants change position several times between shots. At first they are pulled up with her legs exposed, then they are down further, and in the last shot when Ross gets kicked, they are pulled up again.
The One Where Estelle Dies - S10-E15
Continuity mistake: Near the end when Chandler is talking to Janice at one point his hands go from pointing straight ahead to his pockets. (00:17:20)
The One With Rachel's Going Away Party - S10-E16
Continuity mistake: The level of beer in Ross' bottle goes up and down between shots as he's talking to Joey at the kitchen table. (00:12:00)
The One Where Estelle Dies - S10-E15
Continuity mistake: The second time Ross visits Mr. Zelner, he hands him a pterodactyl egg replica. The hole in the egg switches between the top or bottom, depending on the shot. (00:14:40)
The One Where Estelle Dies - S10-E15
Continuity mistake: When Rachel walks in with Emma, Ross is on the laptop. It is straight and on the table, in a later shot,the laptop has moved and is over the edge of the table. (00:18:50 - 00:20:55)
The One With Ross's Grant - S10-E6
Continuity mistake: When Aisha Tyler and Greg Kinnear put one hand on Ross's face, they bring those hands together and they interlock fingers, then the camera changes positions and they interlock fingers again. (00:22:20)
Continuity mistake: When Monica and Chandler walk into the apartment with the twins, 2 very bright yellow soothers keep appearing and disappearing from the babies' mouths. (00:18:50)
The One With Princess Consuela - S10-E14
Continuity mistake: When Ross is trying to get Rachel's chair through the revolving doors, he has his right leg on the chair but it suddenly switches to him having his left leg on the chair. (00:15:25)
The One Where Chandler Gets Caught - S10-E10
Continuity mistake: Ross claims he looks into Monica and Chandler's and watches them eating dinner and watching TV. His apartment is a floor lower down than theirs and with the balcony wall, the sofa and tables distance from the window and the fact the bottom of the window is higher than the floor, there is no way Ross would be able to see that far in. We have seen in the past that he can basically only see who is at the window. (00:19:05)
The One With Princess Consuela - S10-E14
Continuity mistake: When Rachel starts to cry, after Ross brings the champagne, Ross hugs her and holds the glass in his left hand, but when it cuts to the wideshot, the glass is instantly on the table in front of him. (00:10:20)
The One Where Estelle Dies - S10-E15
Continuity mistake: When Chandler says his boss is getting him a new lamp in his cubicle, his arm is over the back of the chair, in the immediate next shot it is moved and then back again. (00:00:50)
The One With The Late Thanksgiving - S10-E8
Continuity mistake: When Rachel and Phoebe are talking in the Baby Beauty Contest and Rachel is holding Emma, Emma's head instantly changes position between shots several times, often going through 180 degrees. Clearly they couldn't get her to do the scene in one take and therefore the editing suffers. (00:07:05)
Continuity mistake: When the friends put down their keys, the keys' positions change when the camera pans the apartment. (00:25:30)
The One With Ross's Grant - S10-E6
Continuity mistake: When Monica comes round to Phoebe's, Phoebe reminds her that she's got to clear out stuff. In this wide shot Monica has both hands by her sides, but when it cuts to her finishing saying "oh right", her right hand is now gripping her bag's strap. (00:05:00)
The One With The Late Thanksgiving - S10-E8
Continuity mistake: The food/sauce stain Joey gets on his shirt (after running into the cart) changes between shots. (00:20:50)
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.