Two Thirty - S4-E24
Plot hole: In this episode Arthur scams Deacon and several other parents for phony piano lessons because he doesn't actually know how to play the piano. However, earlier in the series (Art House episode, S1 - E24) we learn that Arthur can play the piano when he temporarily moves out and after his house warming party Doug says to Carrie, "Who knew Arthur could play the piano like that?"
Plot hole: Spence comes in with a pizza and says, "Let's hit this pizza hard and get out of here." But then later we find out there's a laptop in the pizza box. What if the guys actually wanted a slice? Also, after the guys leave for the movies, Carrie or Arthur would've seen the laptop in the pizza box. (00:05:35 - 00:06:55)
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.