Home for the Holidays - S9-E12
Revealing mistake: When the family is eating the Christmas cookies, look at the top of the screen. In the kitchen, you can blatantly see where the top of the kitchen actually ends, revealing gray panels. The Conner's ceiling never went up this high, so it is definitely set material.
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