As I already explained, this leaves a files you've opened 'just to look at' vulnerable to corruption by cats, errant keystrokes etc.
You cannot know to explictly undo the changes that you do not know happened, and the OS will silently save those changes.
That's a new path to file corruption brought on by the new autosave/versions/file locking system. Admittedly, versions will let you get the original back, if you notice the problem before moving a file to a different drive, duplicating etc., but that's more a matter of chance than certainty.
I had the same problem, perhaps in the Mac OS X 10.3.x time (great OS, besides < 10.3.1), do not know exactly, with manual backups (disk images). I can tell you it was an absolute nightmare. Of course, if you think some important data is still available and it is not available, not on the first backup drive, and not on the second. And several OS generations later, we find out Lion provides still no solution. The only solution are backup drives which you use only every month or year on your Mac, so the OS cannot destruct certain files. If you use (encrypted) disk images for your backups, you can only hope Apple supports them in future OS generations (yes, i know
VFDecrypt and
others exist, thank you).
I think they know, why the allow multiple backup drives in ML. What we need? An open source decryption tool for encrypted time machine backups, so we can restore our protected data (with the correct key), even if OS X 12.0 does not support OS X 10.8-encrypted backups. And we need options in Time Machine, which allow us to use a new backup drive only every week, every month or every year. That would make it nearly impossible to damage older backups.