I've had it happen more than once, I've posted about it before. I know it happened to me when the program was frozen and I killed the process. Hasn't happened in awhile, but I think the freezing at one time was related either to an ad or a program update process.

I keep several backup files, I don't back up weekly but probably every few months and certainly if I add someone's accounts to the program (where it wouldn't be easy to recreate).