Continuity mistake: At the start when Carrie walks into the kitchen she opens the door with her right hand. However from the kitchen angle she is opening the door with her left hand and her right is down by her side. (00:03:10)
Continuity mistake: When Doug and Carrie are talking in the kitchen after Doug tried the Sgt. Salty cereal, you can see the cereal box disappear and then reappear on the table behind them. (00:12:45 - 00:13:35)
Continuity mistake: The box of Sgt Salty cereal alternates from open to closed between shots when Doug and Carrie are talking. (00:13:30)
Continuity mistake: After trying the Sgt. Salty cereal, while talking to Carrie, Doug gets out a plate to make something else to eat. After he puts the plate down on the table, the cereal box moves and switches sides. (00:13:30)
Continuity mistake: Near the end when the Heffernans, Palmers, and "Shmenkmans" are having dinner and are in conversation, Carrie is shown sitting extremely close to Abby, who is on Carrie's left. But in shots in the middle of the conversation, there is significant empty space to Carrie's left.
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.