Since Snow Leopard's release, a few users have reported a scary data loss incident related to using the Guest account. Basically, after upgrading to Snow Leopard with an existing Guest account in Leopard, these users logged into their Guest accounts, then back into their normal accounts, only to find all of their data gone--none of their files were there, all preferences were reset, and all programs acted like they were being launched for the first time ever.
While the number of people affected by this bug is relatively small, the loss of data makes it a very serious issue. Cnet originally confirmed the existence of the bug with Apple, and an Apple spokesperson I spoke to on Tuesday told me the company is "aware of the issue, which only occurs in extremely rare cases, and we are working on a fix."
To better understand the nature of the problem, I set out to replicate it myself... and had no luck whatsoever. From what I've read on the problem, it's supposed to occur when you upgrade from Leopard to Snow Leopard, have an existing Guest account in Leopard, and then log into and out of the Guest account in Snow Leopard.
I have a test partition that was running OS X 10.5.8, so I activated the Guest account there (and tested it to make sure it worked). I then ran the Snow Leopard upgrade. After the upgrade, the machine booted and logged me into my usual account. I then logged out, logged into the Guest account, and used it for a while. I logged out and back into my main account, and found all the data was there. I then tried using the Guest account via Fast User Switching, but still, there was no harm to my regular user account.
Clearly this bug exists, based on the posts in the Apple Discussions forum and Apple's acknowledgment of the issue. However, it doesn't appear (at least based on my testing) that the issue affects everyone who had a Guest account in 10.5. Still, if you're presently using 10.5 and are considering an upgrade to Snow Leopard, there are two simple things you can do to protect yourself until this bug is fixed.
First and foremost, make sure you have a current, usable backup of your machine before you upgrade. This is good advice in general, but even more so any time you're upgrading the operating system. Second, if you do have the Guest account enabled in 10.5, disable it (in the Accounts System Preferences panel) prior to upgrading. After upgrading, you can then re-enable the Guest account, as apparently Guest accounts created in Snow Leopard itself don't cause this data loss issue to arise.
If it were me, though, I'd stay away from Guest accounts in general until Apple fixes the problem--unless you have a compelling need to use them in the near term, the risk seems much greater than the potential benefit at this point.
This story, "Inside Snow Leopard's Guest account data loss bug" was originally published by Macworld.