Other mistake: Carrie describes Susie as "one of her best friends." Earlier in this same season, the episode "Female Problems" was about her not having friends anymore. In fact, in that episode, she spent a whole evening calling any former friend that she could think of. She told Doug that they all had babies, hated her, or she hated them.
Continuity mistake: Carrie says that she "didn't go" to college because Arthur felt that she would be abandoning him. He points out that her mother had just died. According to the episode "Class Struggle" (Season Two), Carrie actually went to college and then her mother died, causing her to drop out. In that episode, she wants to "go back" to college.
Visible crew/equipment: When Doug and his friends are moving the TV back downstairs, in the first shot just as Spence announces his finger is getting crushed, a camera operator with camera are visible on the left side of the screen. (00:12:45)
Revealing mistake: When we see the montage of Doug working as a manager, in one overhead shot you can see the image on the PC is simply stuck on to make it seem like it's on. (00:08:50)
Revealing mistake: Even after receiving the gift basket from the neighbors, the Heffernans are still awakened by their barking dog. When Carrie rushes downstairs Doug stops her at the front door, and the entire left window has been removed (for camera access) from its frame, so there's a large empty hole. (00:13:10)
Visible crew/equipment: While Doug and Carrie are in the kitchen talking about Doug's job, right after Carrie says, "Wait him out 5 or 10 years," in the next shot facing the sink, we can see the end of the kitchen set at the right side of the screen. (00:14:00)
Strike One - S3-E5
Visible crew/equipment: After Carrie discovers she can't take a shower because Doug ripped out the shower tiles, when she walks out of the bathroom keep your eye on the window at the right. Just as Doug lifts the hammer a crew member is visible in the bathroom's window (it's not Doug's reflection). (00:17:15)
Visible crew/equipment: At the start, while Doug and Carrie are in the jewelry store, in the shot of Doug swinging the shopping bags at the overhead surveillance camera, we can see a production camera at the left side of the screen. Then in the next shot, from the POV of the surveillance camera, the edge of the store's set is visible at the right side of the screen. (00:02:05)
Continuity mistake: Carrie says that Deacon does not "drink or smoke." Previous episodes have shown him drinking beer with the guys. Also, in the episode "Friender Bender", it was mentioned that he drank a margarita during dinner at a restaurant.
Visible crew/equipment: Lighting equipment is visible in the lower left corner of the shot of the outside of the Heffernan home. (00:14:42)
Visible crew/equipment: Doug, Deacon, and Danny are at their lockers towards the end of the episode, and Deacon says, "Me and Danny gotta do another half shift tonight." Camera cuts, and a boom mic can be seen at the top of the screen.
Visible crew/equipment: When Doug and Carrie are talking about the sketch of Carrie on the fridge and Doug says, "Man, he's got some skills" (talking about Major), a boom mic is visible at the top of the screen.
Frozen Pop - S2-E15
Visible crew/equipment: When Arthur puts his big red jacket on and goes downstairs, Carrie follows and tape markers are visible on the floor by the door. (00:16:30)
Frozen Pop - S2-E15
Continuity mistake: When Arthur comes in and shuts Doug's window, Doug's hand goes from open to closed between shots. (00:06:00)
Frozen Pop - S2-E15
Continuity mistake: When Carrie is talking to Arthur at the start, Arthur's hands are in his lap in one shot and clasped together in the next. (00:02:45)
Continuity mistake: Deacon states on a few occasions in this episode that him and Doug started at around the same time at IPS. However, in later episodes it's revealed Deacon is senior to Doug by quite a bit in flashbacks to how Doug got his job. (00:10:00)
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.