The One with the Evil Orthodontist - S1-E20
Continuity mistake: When Rachel's in Barry's office her black handbag is hanging over the counter, but after Billy walks in on them and Barry pretends to look in Rachel's mouth, the bag's lying on the counter.
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Monica holds the glass pan of lasagna up over her head to try and find Rachel's ring, when it cuts to the next shot the lasagna is still in the air, but it's different - note burned corners, etc. (00:06:10)
The One With The Metaphorical Tunnel - S3-E4
Continuity mistake: When Ross is singing to Ben about GI Joe, there are two plates in the dish drainer behind him, but when Joey walks in and sits down the white plate has vanished.
The One With Ross's Wedding, part 1 - S4-E23
Visible crew/equipment: When Rachel exits her bedroom and says she's going to London, you can see tape markings on the floor by her bedroom door and the end of the set. Furthermore, you can see a chair which is likely part of the Central Perk set. [Widescreen version]. (00:20:25)
The One With Princess Consuela - S10-E14
Visible crew/equipment: At the very start when Monica mentions swinging, all the stage lights are visible in her glass of red wine. (00:00:20)
The One After the Superbowl, part 1 - S2-E12
Continuity mistake: Brooke Shields throws water at Joey, and he gets a large spot of water on his shirt, but in the next shot the spot is much larger than before. (00:15:55 - 00:16:55)
The One with the Stoned Guy - S1-E15
Continuity mistake: When Chandler says Ross isn't speaking metaphorically, he gestures with his right hand. In the next shot, both his hands are in his pockets. (00:06:05)
The One with the Boobies - S1-E13
Continuity mistake: When Joey finds out that his mother knows about his dad's affair, she tells him she's fine and holds her hands to her chest. In the next shot her hands are outstretched in front of her. (00:17:10)
The One with Two Parts: Part 1 - S1-E16
Visible crew/equipment: Just after the credits, before we see a close up of Ross, a boom mic appears at the top of the screen in the centre of the window on the right. (00:01:30)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Ross arrives at Monica's apartment, both Ross and Monica walk past the beam in the middle of the room. In the next shot as they continue walking they walk past the beam again. (00:07:05)
The One With The Free Porn - S4-E17
Continuity mistake: When Joey is asking Ross why he told Emily that he loves her, Joey's bottle of juice is right in front of him. In the next shot of Joey, there's two bottles - Monica's Diet Snapple and Joey's juice, and in the next shot it's back to one again. Monica's drink appears and disappears throughout the scene. (00:11:25)
The One With The Princess Leia Fantasy - S3-E1
Continuity mistake: In the scene where Janice tells Joey they should spend some quality time together, the position of her hair changes from in front of her shoulders to behind several times between shots. (00:13:20)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: When Ross visits Carol, he hugs her, and puts his hands on her hips. In the next shot, he isn't touching her and has taken a step back instantly. (00:05:10)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: At the end of the episode, when Monica scores a goal against Joey and Chandler, she is playing with the red team and scores with her strikers. In the next shot, a blue midfielder scores the goal. (00:21:05)
The One with the Thumb - S1-E3
Continuity mistake: When Ross said his parents sent their dog away, Chandler is holding onto his right foot with his hand. In the next shot, the foot is gone and his arm is in a different position. (00:01:00)
The One with the Monkey - S1-E10
Continuity mistake: When Ross say 'just a smidge' after Rachel says Paulo named her breasts, Phoebe is holding tinsel and Rachel is looking at Ross. In the next shot, Rachel is looking in the opposite direction and the tinsel has vanished from Phoebe's hands. (00:06:00)
The One With The Racecar Bed - S3-E7
Continuity mistake: In the scene where Joey's teaching soap opera for actors, his name changes from the 'a' being on the line to being above it. The line itself also reaches the far right of the board in some shots, and is short of it in others. (00:06:00)
The One with the Blackout - S1-E7
Continuity mistake: During the Monopoly game (deleted scene), right after Monica says, "we can bring my wheelbarrow, to carry your teeth," we see Ross reading the game manual at the end of the shot. At the beginning of the very next shot, he is suddenly rolling the dice with no manual in his hands. (00:17:00)
The One with George Stephanopoulos - S1-E4
Continuity mistake: When Phoebe talks about the cave in in the mine, Ross has his hands in his pocket behind her. In the next shot his hand is on the sofa. (00:04:10)
The One with the Blackout - S1-E7
Continuity mistake: After the light comes back on and Ross is holding his heart after he sees Rachel kissing Paulo, in the next shot when Joey starts speaking to him his hand is by his side. (00:20:40)
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.