I have learned the hard way to backup everything. I didn't use to do that, but with so many different glitches and bugs, now I have a folder seperate from my game that I keep backups in, so far so good, that I have not had to use them, but I know they are there.
I know in the game I was talking about with the invisible sims, both of them were playing chess. You could see the chess pieces moving and you could hear them talking, but no sims and no plumbob. I only used "resetsim" on the one and they both popped back in. Hopefully it doesn't happen very often. Has not happened since, so not sure what the problem was.