Continuity mistake: In the first shot inside Arthur's house, Carrie and Doug are standing together when Carrie mentions how tough it must be for Arthur. It then cuts to Arthur sitting at the table, with Doug right there sitting beside Arthur. Next shot Doug is still standing with Carrie as she urges Doug to spend some time with Arthur, so this is when Doug walks over to his father-in-law. (00:07:00)
Audio problem: While all four guys are on the bed watching the game, Richie's eating a sub so we hear Doug say, "Hey, Richie, come on," but Doug's lips are closed and not moving in this shot. (00:15:10)
Continuity mistake: When Sara is in the shower you can see in her shadow, creases from her swimsuit. (00:16:30)
Revealing mistake: While Doug is driving the IPS truck he stops in traffic, and then Arthur pulls up alongside. Watch the reflection in the truck's side mirror, because we can see crew members pushing Arthur's car as he takes off. (00:19:10)
Continuity mistake: The piano in the living room is ebony in the beginning of the episode. At the end of the episode the piano is brown.
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.