Revealing mistake: Shortly after the fight with Becky, Roseanne asks Jackie to go across the street to get her something to drink, so she can smoke in secret. After Roseanne lights the cigarette and walks near the register, Jackie's plaid coat can be seen in the door's window as she waits for the next scene.
Visible crew/equipment: In a scene where Darlene and Becky are in the kitchen, you can see the microphone above them for a couple of seconds.
Continuity mistake: When the girls are at the Lobo Jackie has a long scarf in her hair that moves between having the length of the scarf behind her back and then draped on her front left side.
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