I moved a couple into an apartment. They rented it furnished, using the testing cheats option. After playing them for a while I exited the game and went to bed. I tried to play them today, but upon returning to the lot I discovered that all their furniture is gone! They are still in the apartment doing what I left them doing, but they no longer have furniture. What happened?

I think that it's fair to say that this is not a known problem. I don't suppose that you have a current backup? If so, the simplest solution might be to just restore from your backup.

You could try moving them out, then renting the apartment again, to see whether you can still rent it furnished. If you backup first, you won't even lose anything if that fails to solve your problem.

I saw this problem recently on Jessa's let's play. She decorated an apartment 3 times and had vanishing furniture syndrome just like what you discribe on reloading the lot. Turned out to be a problem with a clashing mod. Not sure what mod it was though.

If so, then you might be able to find the problem using the Game Help:Game Problem^wiki FAQ. Unfortunately, it sounds like it can take a while for the problem to show up.

Maybe go ask Jessa what mod she had a problem with. http://littleblondesim.tumblr.com/ask

I ended up just moving them out. It didn't happen with another apartment in the same building, but I haven't moved other sims into the problem apartment though. When I tried to refurnish the apartment (with the sims still living in it), it appears that the space which the furniture occupied was still somehow occupied (invisible furniture?), as I could not place furniture in those places. I had the same problem with a dorm once a couple years ago. When I entered buy mode, the furniture in unoccupied dorm rooms didn't appear, and I couldn't place other furniture in it's place. I had no mods back then, a different computer, and fewer expansion packs.

That sounds like a different problem as her furniture and deco had really gone.