Last week, Microsoft and T-Mobile announced that they lost all their Sidekick users data. Not to be out beat, Apple has joined in as users of the new OS X Snow Leopard are reporting the same. According to Apple OS X related newsgroups, complaints on have been cropping up the last few days due to a major bug in the OS that can reportedly lead to all user settings being reset and most data wiped out from the hard drive.

The flaw is triggered when a user signs into the guest account, be it intentionally or unintentionally, and then attempts to sign back into their regular account only to find that all of their files have been erased. Apparently, an issue in the way the new version of Mac OS X handles guest accounts could cause the computer to seemingly ‘delete’ all of a users files stored in their main account.
The number of users impacted by this bug is uncertain as is the exact cause behind it. Speculation is that something makes Snow Leopard treat the regular account like a Guest account and by default, all data is deleted upon logout. This could be due to a bug in how guest accounts are managed since data and settings are deleted from these accounts upon logout, but also could be from some corruption in the guest account.

Apple has yet to address the issue or reply on the USENET newsgroups, but until the company delivers an update it’s probably a good idea to disable the Guest account altogether. This issue is nothing new: CNet has an article from September 19 which seems to allude to the issues that people are experiencing, as well as offering a possible solution to your problems. Performing regular data backups are always advisable and recommended here.
If this happens to you, immediately restore using your latest Time Machine backup; or other full system backup. Also, go to the Accounts system preferences and disable log-in on 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.