Revealing mistake: When Doug runs into the phone when trying to deliver Carrie's package, it's very clear he doesn't touch the phone with his head.
Continuity mistake: In the scene where Carrie and Doug are in the kitchen and Carrie is explaining why she wants to quit school, she fills a glass of water to the top of the glass. A few shots later, she sets the glass down on the counter without taking a drink and it is less than half full.
Visible crew/equipment: When Doug is talking on the phone, the shadow of a boom mic moves on the wall behind him. (00:00:20)
Visible crew/equipment: When Doug is lying on the floor with the phone on him, the camera zooms in and you can see a boom arm reflected in the phone. (00:19:35)
Continuity mistake: When Doug says to Carrie that he doesn't even know her anymore, Carrie's laptop and books are open over the desk. However when the camera cuts to her standing up, she has got the laptop folded up and her books in her arms instantly. (00:10:35)
Continuity mistake: When Carrie is sitting at the table talking to Doug about not finishing school, her hand is beside the text book and then instantly holding it from underneath in the next shot. (00:10:35)
Continuity mistake: When Carrie is having lunch with the lawyers from her office, she is holding a cup and then instantly holding a napkin in the next shot. The cup and napkin switch again depending on the shot. (00:03:10)
Continuity mistake: When Doug and Carrie are arguing about not having gone out for pizza, Carrie has her pen in her hand, but when it cuts it's now on the desk. (00:10:30)
Answer: Sara hasn't been mentioned in later episodes, and it suggested in later episodes that she never existed, and that Carrie is an only child. Kevin James stated that the producers couldn't think of an good plots to use with Sara, so they left her out.