Yet another case for backing up your data: Snow Leopard
Apparently not only are Sidekick users losing their personal data. Now, in a separate incident, Snow Leopard (OS X 10.6) users are also finding their data fully wiped.
The bug was actually discovered within a week of Snow Leopard's launch back in August, when users found that logging out of their account, into a "guest" account, and then back into their personal account would completely erase the content from their home drive (Documents, Movies, Pictures, Music, Sites).
Though the bug is now more than a month old, it's still claiming victims, as Apple's support forums show. However, Apple has yet to acknowledge the issue and the aforesaid conditions do not faithfully reproduce the bug.
The issue is thought to only affect users who had active guest accounts in Leopard (Mac OS X 10.5), and the only workaround currently is to disable guest login altogether.
Users who have succumbed to the bug are likely to permanently lose their data unless they have performed a backup, so users who have guest accounts and upgraded to Snow Leopard from Leopard are advised to back up their data immediately. Users with Time Machine running simply need to hold down the "C" key when booting their Mac, and then selecting Utilities > Restore from Backup in the event that this bug eliminates their data.