Continuity mistake: Just as Doug persuades Deacon to go with him to Carrie's birthday dinner, his position changes completely. (00:10:20)
Continuity mistake: Doug comes out from the payphone and asks Carrie where did everyone go. When Carrie replies, the way she holds her glass changes between shots. (00:17:50)
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.