The One with the Candy Hearts - S1-E14
Revealing mistake: When Rachel pours the rest of Palo's stuff in the fire, it's clear she pours it on the table not the fire. However, the flames billow up. (00:13:40)
The One with the Boobies - S1-E13
Continuity mistake: When they're all sitting together and having pizza, talking about parents, Rachel has no napkin. When she says: "Just think, in a couple of years we get to turn into them", a bunched up napkin appears beside her on the couch. After Chandler's remark it has disappeared again, then it appears and reappears again between shots. (00:21:10)
The One with the Boobies - S1-E13
Continuity mistake: When Joey's mum explains that every day is like her anniversary, her hands are outstretched. Camera cuts and they are lowered. (00:18:20)
The One with the Boobies - S1-E13
Continuity mistake: The way Rachel is sitting when discussing seeing Chandler naked is constantly changing. (00:07:00)
The One with the Dozen Lasagnas - S1-E12
Visible crew/equipment: When Joey and Chandler walk into their apartment and the table falls over when Joey puts the key on it, the edge of the set and studio lighting are visible at the top of shot. The end of the set wall is also visible at the right side of shot. (00:04:50)
The One with Mrs. Bing - S1-E11
Continuity mistake: When Coma Guy asks who Monica and Phoebe who they are, he has his hands lowered on the bed in one shot and raised outstretched in the next. (00:18:30)
The One with Mrs. Bing - S1-E11
Continuity mistake: The way Joey holds the picture of his mum changes between shots. Note his fingers. (00:15:50)
The One with Mrs. Bing - S1-E11
Continuity mistake: At the hospital, the positions of the balloons Monica brings change instantly. Note the blue one. Its height changes drastically from below to level with the yellow one. (00:11:30)
The One with the Monkey - S1-E10
Continuity mistake: When Phoebe asks the guys if there is something they would like to share with the group, David goes from looking at his friend to looking at Phoebe between instant cuts. (00:05:05)
The One Where Underdog Gets Away - S1-E9
Continuity mistake: When Rachel turns up with her tickets, Chandler has his hand on his hip. As Rachel walks off however, his arm is behind her. (00:12:00)
The One Where Underdog Gets Away - S1-E9
Visible crew/equipment: When Joey walks into Central Perk after the VD poster montage, a tape marker is visible on the floor behind Chandlers spot on the sofa. (00:10:40)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: When Rach asks who the naked picture is of, the way she holds it changes between shots. (00:21:10)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: When Monica is talking to her dad at the table, the way he holds the magazine changes between shots. (00:08:50)
The One Where Nana Dies Twice - S1-E8
Continuity mistake: In Central Perk, Joey says "So, Chandler looks gay huh?" When he does his arms are not crossed. Camera cuts and they are. (00:08:30)
Revealing mistake: When Joey is filming his scene for the movie, look behind the director to see a box saying 'Monica' indicating that it was shot on the Friends set rather than Al Pacino's set. (00:16:30)
Continuity mistake: At the very end when Monica is in bed, her grip on the bed clothes changes between shots. (00:23:10)
Continuity mistake: When Chandler's partner says she's going to be late for Andrew she has her hands in front of her. Camera changes and now one hand is up by her hair. (00:18:15)
Continuity mistake: When Ross is reading the book he shows it to Phoebe who passes it back. Then without turning the pages he passes it to Chandler but the pages are different now. (00:07:20)
Continuity mistake: When Joey asks how the play was, Monica's grip on her program changes between shots. Note the gap between her hands. (00:04:20)
The One with the Thumb - S1-E3
Visible crew/equipment: At the end, just as Monica walks off a tape marker can be seen on the floor where she was stood. This is as the credits are on screen. (00:22:30)
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.