The One with All the Poker - S1-E18
Continuity mistake: When Chandler and Ross are explaining how Joey cried during their poker game, Joey's coffee mug is blue when he's holding it at the counter, but when Joey walks away from the counter his mug is yellow.
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: When Ross, Chandler and Joey are setting up Ross' furniture and Ross is bent down saying, "And I cannot feel my legs," Ross is holding up a manual. When it cuts, the manual is suddenly lying on the floor. [In other versions, such as Netflix, the first shot shows the manual falling to the floor and the second shot does not exist. However, the mistake is valid for the TV broadcast version of the episode]. (00:11:45)
The One with the Blackout - S1-E7
Revealing mistake: When Rachel first meets Paolo she's looking for the cat in candlelight, holding the candle in front of her. You can see her shadow on the wall, as well as the shadow of the candle, revealing that the light is not coming from the candle. (00:07:25 - 00:14:30)
The One with the Sonogram at the End - S1-E2
Continuity mistake: When Rachel loses the ring near the beginning, Chandler and Joey get off a chair to help look. But then the shot cuts to Monica, and behind her you can see Chandler at the fridge without enough time to get there. (00:05:40)
The One with the Thumb - S1-E3
Factual error: The show is set in New York. Joey was a cast member on Days of Our Lives, which is filmed in Los Angeles, but is still always around.
The One Where the Monkey Gets Away - S1-E19
Continuity mistake: After Marcel has escaped from the basement, he is eating a banana as a pair of arms reach out to grab him. These arms are supposed to belong to Mr Heckles, their weird neighbour, however the arms belong to someone young, large and hair free. (00:16:10)
The One with the East German Laundry Detergent - S1-E5
Visible crew/equipment: When the horrible woman enters, the boom mike is visible in the mirror at the top of the screen. (00:10:10)
The One with the Dozen Lasagnas - S1-E12
Continuity mistake: When Rachel is talking with Phoebe about Paolo,. she unrolls some kitchen paper and uses it as a tissue. Initially she's holding two torn halves attached to the roll, then when saying "He's a pig" it goes to a whole sheet not attached to any others, then when she sits down it's a whole sheet attached to the roll again. Plus when Phoebe sits too, in some shots the paper is on the table, in others it's not. (00:15:20)
The One with George Stephanopoulos - S1-E4
Continuity mistake: When Rachel gets her first paycheck, the envelope is already open, but she then rips it open. (00:04:20)
The One with the Boobies - S1-E13
Continuity mistake: We've seen the arrangement of Joey and Chandler's bathroom in two previous episodes (1x3, The One with the Thumb, and 1x12, The One with the Dozen Lasagnas) with the sink opposite the bathroom door. In this episode the sink is relocated and the bath/shower is now opposite the bathroom door, for the rest of the series.
The One with the Ick Factor - S1-E22
Continuity mistake: When Monica takes off her belt and throws it onto the couch, it lands on Rachel's arm. Throughout the scene, between shots of Rachel and Monica talking, the belt constantly disappears, reappears and changes position without Rachel ever touching it. (00:05:25)
Revealing mistake: After Chandler asks the woman out, he runs back over to the group shouting, "She said yes! She said yes!" As he does this Joey can be seen mouthing the line. (00:03:45)
The One with the East German Laundry Detergent - S1-E5
Visible crew/equipment: While Chandler is trying to break up with Janice, and Phoebe comes to help, a cameraman is visible in the background (outside of Central Perk) walking from right to left. The camera cuts away from the window angle, then cuts back, showing another cameraman walking from left to right. (00:19:15)
The One with the Thumb - S1-E3
Continuity mistake: When we first see Chandler in his office, his sleeve is rolled below his elbow. In the next shot, it is above his elbow. (00:12:30)
The One with the Evil Orthodontist - S1-E20
Other mistake: This is the episode where a guy across from Monica's apartment is spying on them. Joey says he caught him spying into his and Chandler's apartment yesterday. This is impossible considering the layout of the building i.e. the fact that the only part of Joey and Chandler's apartment which faces toward the spying guy has the stairwell and hallway blocking it from view (not to mention the protruding part of Monica's apartment). There is another window in Joey's apartment but it faces completely the wrong direction. The bathroom has no windows. The angles just don't add up. (00:00:40)
The One with the Blackout - S1-E7
Continuity mistake: Rachel takes the cat to apartment number 8 and Mr. Heckles opens the door. However, in this episode, the girls' apartment is labeled number 5 which means that number 8 is above them. In later episodes, Mr. Heckles always says that the noisy girls upstairs are keeping him from doing things. [It makes a bit more sense later on in the series when Monica and Rachel's appartment changes to number 20, which is when Mr Heckles starts to complain about the noise (although in the flash back episode when Phoebe is still living with Monica it seems apparent that they actually know Mr Heckles, which Phoebe shows no sign of when she and Rachel are looking for the owner of the cat).] (00:13:40)
The One with the East German Laundry Detergent - S1-E5
Character mistake: In the laundromat, there is a sign where the word clothes is misspelt cloths. (00:16:00)
The One Where Monica Gets a Roommate - S1-E1
Continuity mistake: While Joey and Chandler are at the apartment with Ross, a watch suddenly appears on Joey's wrist when he announces that he and Chandler are done with the bookcase, then the watch vanishes and reappears again in the following shots at the apartment. (00:15:40)
The One with George Stephanopoulos - S1-E4
Visible crew/equipment: When Monica's looking out the window at George Stephanopoulos with the binoculars, the end of the set wall and overhead studio lighting can be seen at the left side of the screen. (00:15:20)
The One Where the Monkey Gets Away - S1-E19
Other mistake: When Rachel's watching the soap opera on TV while babysitting Marcel, she and Marcel are sitting in the middle of the couch, but in the closeup of Dr. Francis we can see the reflection of the empty couch at the bottom of the TV screen.
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.