Bug in OS X Snow Leopard Deletes All User Data
Users of Snow Leopard, Apple's latest Operating System, who have the guest account enabled seem to be affected by a rather unpleasant bug, as more and more reports on the Apple Support forums indicate.
Already a few days after its launch back in September users started reporting problems when they logged into the guest account and then back into their normal one. After this settings could be reset and data gone, "as though it were a brand new account."
"The home directory still exists under "/Users/username" but is completely empty. Users are reporting that the data is unrecoverable and cannot be found anywhere on the hard drive, and the only way to restore it is if the user has been performing backups on a separate hard-drive."
Currently there seem to exist two possible work arounds, though:
"This problem does not happen on my Snow Leopard machine (upgraded from Leopard) upon enabling guest log-in, which suggests one possibility could be some Snow Leopard incompatibility with how the guest log-in was initially enabled. If you had it enabled in Leopard before upgrading and the problem happened without any changes to the guest account settings, then it could be from an incompatible setting between how Leopard and Snow Leopard handle the guest accounts. As such, try going to the Accounts system preferences and toggling the guest log-in setting to see if that refreshes the way the system is handling the guest account.
If you need guest account functionality and do not trust the built-in account because of this problem, for now just create a new non-administrator account (call it "Visitor" if you need a semi-decent alternative name) for use as a guest, and customize restrictions for it with parental controls. In most instances this will work just fine, since the only real difference in behavior for guest accounts is that data and settings are reset upon logout."