The fault was first noticed shortly after the OS's release
Users of the new Apple operating system Snow Leopard are experiencing massive data losses when logging into their machines under a guest account.
The problem appears to affect those who had a guest account enabled before upgrading to Snow Leopard.
Users have in some cases lost their entire main profile, including sites, pictures, videos and documents.
The problem, reported by more than 100 users on discussion forums, surfaced shortly after the OS's August release.
The issue follows closely on the heels of vast data losses by the Sidekick handset in the US, whose software was designed by Microsoft subsidiary Danger.
Indications are that the Snow Leopard bug simply treats the principal account like a guest account - meaning that the account profile is wiped clean when logging out.
Users who first log into a guest account and then into their normal account have found it to be completely reset to factory default settings, with none of their personal data or files visible.
"I've been using Macs for decades...what the heck have I done here?" wrote user Wingrove on the Apple discussion forums on Monday.
"Repeated restarts and logins never get me back to me usual user acc(ount)."
Those who use backup services including the Mac's own Time Machine can restore their lost data, but it appears that for those who do not, the data is permanently lost.
Apple said in a statement that the problem "occurs only in extremely rare cases" and that it was working on a fix.
In the meantime, users should delete previous guest accounts and, where necessary, create new ones that are "native" to Snow Leopard.