I performed a backup of my Lightoom 4.2 catalog, which was 1.2 GB in size, upon exiting an editing session. No errors were reported. The next time I opened Lightroom it said the catalog was invalid. Upon checking via Windows Explorer I found both the catalog and it's backup to be totally empty -- 0 bytes in size. It appears to have deleted the data in the catalog during the integrity checking/optimization process and then the resulting empty catalog was backed up. Adobe support concurs with this judgement but has no explanation as to how it could have happened. I have no recourse but to revert to an earlier backup which will cause me to lose a week's worth of work. An error as catastropic as this should not occur.