The One With The Tiny T-Shirt - S3-E19
Continuity mistake: When Joey and Kate are talking for the first time, Joey is holding a mug. Between shots, sometimes he is holding the mug by the handle and sometimes by the mug, with the handle sticking out. (00:04:40)
The One With Two Parties - S2-E22
Continuity mistake: When Chandler and Joey are in Monica's apartment blowing up balloons, Chandler's balloon changes type. The camera changes and it goes from shiny to dull. (00:03:30)
The One With The Rumor - S8-E9
Continuity mistake: When Will is demanding yams, it shows Ross looking towards the door; the camera then cuts to Will, and Ross is now looking at Will without time to move his head. (00:11:55)
The One The Morning After - S3-E16
Continuity mistake: In previous episodes we've seen that from the living room there's a short wall just inside Monica's bedroom (presumably to prevent a clear view inside, as her bedroom set isn't normally there). However in this episode when most of the gang are in her room, they're listening right by the door into the living room with nothing in the way. (00:11:25)
The One With The Vows - S7-E21
Continuity mistake: After Monica says "Oooh can we lie in our vows?" it cuts to Rachel then back to Monica leaning on her hands, but when it goes for a close up she is holding the pen in her right hand, there isn't enough time for her to naturally pick it up. (00:03:10)
The One With Ross's Wedding, part 2 - S4-E24
Continuity mistake: When Felicity finds Joey outside the wedding venue she closes the door. But when they start kissing, the door is completely open. (00:35:50)
Continuity mistake: Near the end, when Joey is asking Ross if Bonnie's room will be free tonight, he is pointing his hands at Ross. In the very next shot they are in his pockets. (00:20:50)
The One With Joey's Fridge - S6-E19
Continuity mistake: When Joey pushes Ross against the fridge, a box falls from the top of the fridge onto the shelf beside it. A few seconds later when Joey turns to Chandler, we can see over his shoulder that the box is back on the fridge. (00:08:55)
The One With Joey's New Girlfriend - S4-E5
Continuity mistake: Chandler is chasing Kathy down the street trying to get her attention. During a close-up of him running, just before the camera cuts away we see a guy's face on the left; when it cuts back to Chandler he runs for a while and then encounters the same guy (the dog-walker). Also, when he finally lands in front of Kathy, there is a bike rider coming towards them, but as the camera cuts back and forth between the two, the rider never passes them. (00:11:30)
The One Where No One's Ready - S3-E2
Continuity mistake: The amount of fat in the glass differs throughout the length of the episode. (00:00:05 - 00:15:00)
The One With Joey's Fridge - S6-E19
Continuity mistake: The handle to the fridge door is on the left, so when it opens the inside is visible. In earlier episodes the handle was on the right and it opened out towards the room, blocking the inside from view. (00:00:05)
The One With The Video Tape - S8-E4
Continuity mistake: When Chandler and Monica are trying to call Greg and Jennie, the water bottles on the table move around. Phoebe and Joey didn't move them, they are drinking from the cans. (00:05:30)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: During the scene where Phoebe is telling Rachel that Paolo made a pass at her, Rachel unrolls a long length of kitchen roll. During the scene there is a shot over her shoulder where we see that the kitchen roll is as normal and in the next shot we see it unrolled again. (00:15:30)
The One With the Holiday Armadillo - S7-E10
Continuity mistake: When Ross is on the couch telling Ben about Chanukah, Ben's hair varies from shot to shot. In the first shot it's flat and in the next shot it's spiked up at the front. His hair stays spiked up for a few shots and then returns to flat. (00:06:00)
The One With The Apothecary Table - S6-E11
Continuity mistake: The red eraser knob on the magnadoodle changes from one side to the other, when Janine is going out to tell Monica she is sorry. (00:18:00)
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: After Ross asks Marcel to bring him the rice, he picks up his chopsticks. In the next shot, he is holding a napkin. (00:02:15)
The One With The Video Tape - S8-E4
Continuity mistake: As Rachel goes to put the video on, Ross hangs his head. The camera cuts and now it isn't hanging. There was no time to move it between camera cut. (00:16:10)
The One Where Ross Got High - S6-E9
Continuity mistake: When Ross finishes the trifile, a small amount of whipped cream appears and disappears throughout a few shots. (00:16:35)
The One with the Fake Monica - S1-E21
Continuity mistake: When the girls are at the dance club Rachel has a watch on, and it appears and disappears throughout the scene. (00:05:15 - 00:06:15)
The One With the Male Nanny - S9-E6
Continuity mistake: When Ross storms into the kitchen, exasperated by Sandy, he is immediately followed by Rachel - but she had just been holding Emma, and there wasn't enough time for her to have given Emma to Sandy before storming into the kitchen. (00:25:00)
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.