Other mistake: When Monica's trying to evoke some kind of an emotional reaction from Rachel, who's polishing her toenails, there's a bottle of Cutex nail polish remover on the table with part of the brand name painted black, so it reads "Cu Ex" on the label.
Continuity mistake: After Joey tells Chandler about his identical hand twin and plans for getting rich, when he sees Chandler's reaction Joey asks, "Are you unsupporting me again?" while he lifts his left arm and points accusingly at Chandler, but next shot it's Joey's right arm that's up and pointing.
Visible crew/equipment: While Joey's looking for the "hot girl" the second time, after he knocks on the little girl's door and asks if her mother, sister, or babysitter are a hot girl, he runs off when she shouts for her father, and when Joey stops in another hallway full of doors the shadow of the moving boom mic can be seen on the door beside him.
Visible crew/equipment: After Monica tells Rachel that a guy from Ralph Lauren called and they want her for a second interview, when Rachel walks toward the TV and then when she stands in front of the TV, we can see a few tapes marks on the floor. One of those tape marks is also visible in one of Joey's closeups.
Continuity mistake: When Phoebe gives the housewarming gifts to Ross and also when Ross opens the door for the neighbor, his apartment number is "3B" and his neighbor's door is "3A," but in episode 14, "The One Where Everybody Finds Out," the door can be seen three times during the episode, and the number is "201" each time.
Visible crew/equipment: After Monica and Chandler leave the apartment to go "do laundry," Rachel and Joey tell each other that they can't share what they each know, and just as Rachel sits down beside Joey we can see the movement of camera equipment in the window's reflection.
Visible crew/equipment: When the passerby drops his lit cigarette in Phoebe's charity bucket, the end of the department store facade can be seen at the left side of the screen, and later when Phoebe hands off the bell to her replacement the end of the right side facade can be seen as well.
Visible crew/equipment: While everyone's in the apartment Ross is on the balcony talking to Emily on the phone, and in the mirror we can see the reflections of Phoebe and someone else wearing a blue shirt (with a large white pattern) and blue pants (looks like jeans), but none of the others are actually dressed like that.
Visible crew/equipment: While Rachel and Ross are at the apartment, Rachel tries to convince Ross to have brunch with her father so they can get along, and in the shots facing Rachel we can see the movement of a crew member in the window's reflection.