The answer for now is not to log on as a 'Guest' account. Snow Leopard users have reported that they've lost all their personal data when they've logged into a 'Guest' account after upgrading from Leopard, according to messages on Apple's support forum.
The bug, users said in a well-read thread on Apple's support forum, resets all settings on the Mac, resets all applications' settings and erases the contents of critical folders containing documents, photos and music.
The MacFixIt site first reported the problem more than a month ago.
Users claimed that they lost data when they'd logged into their Macs using a 'Guest' account, either purposefully or by accident. Reports of the bug go back to Sept. 3, just six days after Apple launched Snow Leopard , or Mac OS X 10.6. Users who said they'd encountered the bug said that they had upgraded their systems from Mac OS X 10.5, known as Leopard.
Specifically, Snow Leopard's home directory -- the one sporting the name of the Mac's primary user -- is replaced with a new, empty copy after users log-in to a Guest account, log out, then log-in to their standard account. All the standard folders -- Documents, Downloads, Music, Picture and others -- are empty, while the Desktop and Dock have reverted to an "out-of-box" condition.
"I had the Guest account enabled on my MacBook Pro," said a user identified as "tcnsdca" in a message posted Sept. 3. "I accidentally clicked on that when I went to log in. It took a few minutes to log in, then after I had logged out of that account and back into mine, my [entire] home directory had been wiped. All of doc, music, etc. gone."
"Add my parents to the list of people waxed by this bug," added "Ratty Mouse" today on the same thread. "Brand new iMac, less than one month old, EVERYTHING lost. Just as I convinced them to go Mac after years of trying."
On the thread, several users urged others to disable any Guest accounts to prevent any accidental data loss.
Some people were able to restore their Macs using recent Time Machine backups, but others admitted that they had not backed up their machines for weeks or months.
"Just my luck I hadn't made a backup since 11th August," acknowledged "rogerss" on a different support forum thread. "So annoyed now, in the process of restoring from Time Machine, but have lost loads of my work due to this fault."
Others users, however, had neglected to back up their Macs.
"Nooooo!!! This morning I had access to Guest Account and than all my data were lost!!!" bemoaned someone tagged as "carlodituri" last Saturday. "I had 250GB of data without backup and I lost everything: years and years of documents, pictures, video, music!!! Is it possible to recover something? Please help me!!!!"
Not surprisingly, users unaffected by the bug were reluctant to attempt to reproduce the problem. Some, for instance, wondered if the data loss would be triggered on Macs upgraded to Snow Leopard when the Guest account was simply set to "Sharing only," which is the default.
Apple did not respond today to questions about the bug.