According to many posts at Apple’s discussion forum, a major bug is loose in their newest operating system, Mac OS X 10.6 Snow Leopard. The bug is quite serious, because it deletes all user data and you have no opportunity to restore the data if you haven’t backed up your files in Time Machine or similar.
The bug is reported by several users and it appears only if you have upgraded from Mac OS X 10.5 Leopard AND had a guest account that you haven’t deleted before upgrading or after. If you have upgraded from Leopard and still have a guest account, then DO NOT access it and change back to your user account, because this activates the bug that will erase all of your data in the user directory.
It’s recommended to deactivate the guest account for a while until the bug has been fixed in a software update. If you still wants a guest account, then create an account with limited permission.
No official announcement from Apple has been posted on this bug, that appeared the first time on Apple’s discussion forums just few days after the release of Mac OS X Snow Leopard.