Revealing mistake: In the final scene, Carrie starts to kiss Doug and pushes him onto the bed. The camera angle switches from what would be outside the window to beside the bed. The scene was done in one take from 3 camera angles. As the camera pans left you can see the center window is completely open because they had just pulled the camera out of it and didn't get it closed yet, and you can watch the window be closed by an invisible hand from underneath. (00:28:00)
Visible crew/equipment: At the end of the episode when Carrie and Doug are arguing about her letting him win a ping pong game, all the windows over the bed are closed, they leave the bedroom to go play a rematch, when they return to the bedroom Carrie starts pushing Doug down on the bed and the middle window is open, as they lay down further you can see someone swing the window shut. (00:27:00 - 00:28:00)
Continuity mistake: When Arthur asks Doug for a game of ping pong, a ball suddenly appears under his bat and then disappears and Doug throws him the ball to have the first serve.
Continuity mistake: When Arthur asks Doug for a game of ping pong, the paddle is lying flat on the table. In the next shot there is a ball under the paddle which Arthur picks up (The ball is still on the table). In the next shot the paddle is back on the table and the ball is missing. (00:03:15)
Revealing mistake: When Doug snaps his bat it's very clear it is pre-broken. (Possibly from a previous take or to make it easier for Kevin James.) (00:07:55)
Continuity mistake: In the scene where Spence is entering the place his girlfriend works, he is pushing the door in. When the camera angle switches he is pulling the door open.
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.