Revealing mistake: When Doug and Carrie are waiting in line at the DMV, a man (casting extra) can be seen just outside the doorway waiting to enter the scene. When the shot changes and Doug and Carrie approach the counter, the man casually enters the set. (00:03:30 - 00:04:00)
Revealing mistake: During both opening and closing driving scenes, a light on the back and top of Doug and Carrie's heads slowly flickers on and off but never moves - revealing a stage light simulating driving past street lights. (00:00:20)
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.