Visible crew/equipment: Jessica asks Carrie what number they live at - as she does so a tape marker is visible on the floor. (00:06:50)
Continuity mistake: When Doug and Arthur are sat at the dinner table, Arthur's hands go from on the table to clasped between shots. (00:07:45)
Other mistake: When Doug and Carrie go to visit the new neighbour, Jessica, Carrie tells her that they live at #8013; however many other episodes we see the numbers 3121 on the Heffernan's door and later learn that the Heffernan's live at 3121 Aberdeen Street in Rego Park. (00:28:35 - 00:29:35)
Revealing mistake: During the scene when Doug is "out with the boys" driving in Spence's car, you can tell they are actually in a studio using a green-screen because the stage lights reflecting off the car don't move. (00:36:00 - 00:36:35)
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.