Continuity mistake: The lady walks away from Carrie's desk with some flowers. When she does, there are three petals on Carrie's desk which disappear without being touched. (00:06:10)
Soft Touch - S2-E22
Revealing mistake: When Carrie is trying to park, the position of the shadows on the ground is changing drastically between shots. This cannot be attributed to it taking Carrie a long time to park, as the shadows move forward and back. (00:02:00)
Strike Too - S3-E6
Continuity mistake: The writing on the blackboard changes between shots when Doug writes his name. Note the R in Mr. (00:15:10)
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.