The bug seems to affect users who had a guest account enabled on their Mac machines prior to upgrading Snow Leopard.
Some forum postings suggest that Mac users have lost their entire main profile when upgrading to Snow Leopard, including websites, pictures, videos and documents.
According to Apple Computer, the Snow Leopard bug treats the main account in Snow Leopard as a guest one, meaning that the account profile - and its associated data - is effectively wiped from the machine when the user logs out.
PCWorld.com has reported that Mac users who first log into a guest account and then into their normal account have found their normal Snow Leopard user account is completely reset to factory default settings, meaning that their personal data files are no longer visible.
Postings on the Apple support forum suggest that the data is not normally wiped from the hard disk, just the header information, but retrieving that data is a highly complex task.
Infosecurity notes that Mac users who use backup services and software - such as Apple's Time Machine, which operates in a similar fashion to the Windows Restore function - can recreate the files, but users must have installed the backup software before hitting problems.
Apple Computer said in a carefully worded statement that the problem "occurs only in extremely rare cases" and that it was working on a fix.
Users are advised to delete previous guest accounts and create new guest IDs that are native (i.e. created within) the Snow Leopard environment.