The One with the Dozen Lasagnas - S1-E12
Audio problem: The very last line of the episode (Monica saying "and I'm gone") is an overlay. (00:22:40)
The One with the Thumb - S1-E3
Audio problem: When Monica is first seen in the kitchen complaining about taking a guy home, the other chef is cutting some items up. However, the audio of the knife clicking as it hits the board is overdubbed. (00:08:00)
The One Where Monica Gets a Roommate - S1-E1
Audio problem: When Phoebe says "I just don't want her to go through what I went through with Carl" the camera cuts to Chandler's reaction. When you look at the bottom Phoebe's mouth is moving even though she's not speaking. (00:01:39)
The One With Barry & Mindy's Wedding - S2-E24
Continuity mistake: Phoebe takes out her gum before kissing Joey. After she kisses him, she makes a comment about it and her mouth is empty. In the next shot, she sits down and is chewing. But she doesn't put the gum back in. (00:02:22)
The One With Frank Jr. - S3-E5
Other mistake: Monica doesn't realise what the floor looks like under the hamper, but due to her obsessive cleaning she would know this for sure.
The One With Ross' Step Forward - S8-E11
Other mistake: In the opening scene when Rachel says "I went from store to store sitting on Santa laps" the camera is out of focus.
The One With the Holiday Armadillo - S7-E10
Revealing mistake: When Ben sits on the sofa to listen to Ross talk about the festival of lights, you can see Monica move Ben to the other side of the sofa despite there being space for her to sit. This is so they sit in the correct order as she would mask Ben to the cameras.
The One In Barbados, part 2 - S9-E24
Continuity mistake: As Mike and Monica are playing ping-ping, two people playing checkers magically materialize behind Monica. One second they're not there and the next second they are. (00:12:25)
The One With The Free Porn - S4-E17
Other mistake: 10:50pm boarding and departure, but it is broad daylight outside. (00:09:25)
The One With The Blind Dates - S9-E14
Continuity mistake: When Ross and Rachel are at the coffee shop after their dates, Rachel's necklace disappears and reappears twice. (00:20:20)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Ross shows Chandler and Joey the can of "Carol's favorite beer" it's unopened as Ross makes a crack about how he "should have known," but in the next shot we see Ross lifting the can and drinking from it, even though he never actually pulled open the stay-tab between the two shots (viewed in widescreen). [In the extended version of the episode you see Ross open the can before taking a drink. In the standard version, the mistake is valid]. (00:09:55)
The One With The Giant Poking Device - S3-E8
Continuity mistake: When Ben says Monica's name she's holding the Minute Maid juice box, but in Ben's closeup the juice box is on the table, though it's back in Monica's hand in the next shot. (00:08:50)
The One Where Monica Sings - S9-E13
Visible crew/equipment: After Joey dips his finger into the hot wax and smears it all over his hands, when Sonya walks out of the back room we can see her tape mark on the floor, which she promptly steps on.
The One After the Superbowl, part 2 - S2-E13
Visible crew/equipment: When Joey walks into the bathroom and starts whistling, the shadows of the camera and a passing crewmember can be seen on the door of Chandler's cubicle, then the camera's shadow moves as Joey walks toward Chandler and also when Joey's about to stand on the toilet.
The One with the Monkey - S1-E10
Continuity mistake: When David kisses Phoebe in the lab the first time, he writes "yes" on the board. Later on when they are in the lab again the "yes" is different than the previous one.
The One Where Ross Hugs Rachel - S6-E2
Visible crew/equipment: After Ross tells Phoebe that he does not have feelings for Rachel, just as Ross walks into the apartment to find Rachel crying, the shadow of the moving boom mic can be seen on the wall at the top, left side of the screen. (00:20:15)
The One With Ross and Monica's Cousin - S7-E19
Continuity mistake: When Monica comes into Phoebe's apartment for her Surprise Bridal Shower, the view of the hallway outside the apartment is a dead end on the right side (the left side cannot be seen), however, in S7:E12 (The One Where They're Up All Night), both when Phoebe is disposing of the fire alarm and when the fireman is at the door, it is clear that there is another apartment opposite hers, as well as a trash chute on the right side. (00:18:25)
The One Where They're Up All Night - S7-E12
Revealing mistake: When Chandler is getting a pan out to warm some milk, he successfully pulls the bottom pan out without too much disruption, however, the joke (and entire plot device - to wake Monica up) is to make as much noise as possible so Perry pulls out more pans on purpose and knocks them on the floor. This is quite clear - you can see him grasp the wok and pull it out onto the floor. (00:06:30)
The One With The Lesbian Wedding - S2-E11
Continuity mistake: 12 hours before the wedding, Monica asks Joey to speed it up. Here she is stirring a bowl. In the next few seconds you can see Monica holding some condiments in her hand. After Phoebe answers, Monica is stirring again and the condiments are nowhere to be seen.
The One With The Blind Dates - S9-E14
Continuity mistake: When Phoebe and Joey are at the coffee house discussing their plan to ruin Ross' blind date just before Joey says, "Ooh I hear that's bad" the pendant on Phoebe's necklace is smooth and blue. When the camera pans to Phoebe after Joey says his line the pendant is flipped over and reveals a new design. This continues to happen throughout the scene. (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.