We are aware of the issue, which occurs only in extremely rare cases, and we are working on a fix, an Apple representative said in a prepared statement released to CNET on Tuesday.
The bug has been related to guest accounts and Snow Leopard upgrades. Unfortunately, using a guest account has the potential to destroy personal data, though Apple has acknowledged that it is aware of the problem and is working on a solution.
I accidentally clicked on my guest account and after I logged out, my documents folder was gone, one users said on the Snow Leopard forum. Not only did Snow Leopard wipe out all of my documents, my email accounts, my address book, it broke the dynamic spell checker in Yahoo Messenger, caused random problems with Safari, InDesign, and others.
The bug seems to afflict only Macs that have been updated to Snow Leopard from the previous operating system, Leopard.
The problem was first noted in September by MacFixIt as users who experienced the problem began posting about the issue in Apple's support forums. Later, MacFixit identified a way to avoid the problem: disable the guest account before updating, and re-enable it after updating. Or better yet, if you don't really need guest accounts, just leave them turned off.