Strike One - S3-E5
Revealing mistake: In the kitchen, when Doug tells Carrie he knows the strike will end in 1 or 2 weeks tops because he's a Union Member, the blue shelves/cabinet on the wall behind Doug are positioned properly. Then, when Carrie asks him what the strike issues are, watch as the blue shelves move sideways behind Doug (this wall with blue shelves/cabinet is hinged to provide camera access). (00:13:00)
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.