Revealing mistake: When Deacon tries the doors on the van, the wall moves, revealing it's not a van but a set. (00:08:30)
Revealing mistake: In the final scene of the episode, Arthur is searching for used bottles of nasal spray. He empties the contents of the trash can on the kitchen floor and finds one after sifting through the trash. However, the bottle had already been positioned on the floor next to the trash can before he picked up the trash can so it would be easy to find when the trash was emptied on the floor.
Window Pain - S5-E2
Revealing mistake: When Arthur jumps over the turnstile at the subway where Spence works you can see it is a stuntman wearing the same outfit as Arthur. (00:37:15)
Revealing mistake: When Carrie knocks the cards over at the start, some stay in the pyramid shape despite being knocked over, revealing the cards are glued together. (00:03:15)
Revealing mistake: Both times when you see an overview of the bridge, it's the same clip being used. (00:07:10)
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.