Head First - S1-E6
Revealing mistake: When Doug is making breakfast it's clear there is no flame coming from the gas hob. (00:08:15)
Continuity mistake: At the start, Carrie is saying she wants popcorn with the movie and is motioning with 1 hand. Camera cuts and now both arms are outstretched. (00:00:30)
Continuity mistake: While Arthur tries to convince Spence about the card, when Spence bites into the fake cookie he's holding it in his left hand and holding the plate in his right hand, but in the next shot facing Arthur the items have switched hands. (00:16:30)
Continuity mistake: When Doug and Carrie are told that mold isn't covered by their insurance, Doug has his fist clenched by his mouth. Camera cuts and now it's open palmed and on the side of his head. (00:01:00)
Continuity mistake: When discussing the Superbowl at IPS, Danny has a roll in his hands. Camera changes and he picks it up again. (00:04:30)
Continuity mistake: When Carrie starts praying in the bedroom, Doug has his hands together. Camera changes and he has got his watch in his hands. A watch that was not in his hands or on his wrist before, might I add. (00:09:50)
Continuity mistake: Arthur gives Holly a book at the start and she has it in her right hand. Camera cuts and now it is in her left hand. (00:03:20)
Continuity mistake: When the doctor is explaining to Carrie what is wrong with Arthur, he has a stethoscope round his neck that keeps changing position between shots. (00:08:30)
Continuity mistake: When Arthur and Spence are talking in the kitchen, a fork keeps appearing and disappearing from Spencer's plate. (00:05:40)
Continuity mistake: When the Marshall is chasing after Doug the distance between the two changes drastically. (00:16:00)
Continuity mistake: When Doug is being a polar bear, the bag Carrie was looking in disappears. (00:08:00)
Continuity mistake: When the guys are having brunch, Kelly has got her mug in her hand. Camera cuts and her grip has changed on the mug completely. (00:06:40)
Continuity mistake: When Carrie is serving up the burgers for dinner, she places one on Kirby's plate. Camera cuts to Kirby sitting down and the burger has vanished. (00:10:25)
Strike One - S3-E5
Continuity mistake: When in the garage, the way Carrie's arms are crossed changes instantly. (00:09:50)
Continuity mistake: At the start of the episode when Arthur is on about kissing ass, the way his napkin is positioned constantly changes. (00:03:45)
Continuity mistake: At the beginning Carrie is dishing out potatoes from a bowl with a large black spoon, and as the camera changes angles her arm has not moved and she is now holding a small fork getting chicken from a different bowl. (00:01:40)
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.