Revealing mistake: When Dan calls Becky, there is a shot of her and Mark in their Minneapolis apartment. This is the exact same set that is used later for Darlene's Chicago apartment when she goes to college. The fireplace shape and style, the wallpaper, position of the bathroom door, and the windows are exactly the same. The only difference is that the Darlene version looks shabbier.
Revealing mistake: After Charlotte picks out a dress with Roseanne in the kitchen, and she tells Darlene that she should also go to the prom, she runs from the kitchen like she is going home, but she is running between the kitchen and outside wall of laundry room. (00:08:00)
Answer: Clearly, the only person who could truly answer this question would be a set designer for the series, or someone else directly connected to the series who would have this kind of detailed information. I looked on IMDb.com and Cheryl Lanner is credited as set designer through 1997. I quickly found her Facebook page, and I directed your question to her. After some time, Ms. Lanner has replied as follows: "I decorated the last season 1/2 but after 9 seasons the sofa just plain got worn out and had to be repaired from 8 years of usage. There was a board put on the sofa but as I've said, it was due to so much usage. If they had back problems I was unaware of them, but you can be assured that if that was the problem it would have been fixed immediately. Take care Michael!"
Michael Albert