By Philip Elmer-DeWitt
November 10, 2009

[UPDATE: On Monday, Nov. 10, nearly a month later, Apple released Mac OS X 10.6.2, an extensive update that fixed the guest-account bug and more than three dozen other Snow Leopard problems. For a full list, see here.]

Call it fallout from the Sidekick fiasco.

Having watched Microsoft (MSFT) go through a weekend from hell for wiping out the personal data of thousands of T-Mobile (DT) customers, Apple (AAPL) finally acknowledged a data-swallowing bug that Snow Leopard users have been complaining about since September.

“We are aware of the issue, which occurs only in extremely rare cases, and we are working on a fix,” an Apple spokesperson told CNET‘s Erica Ogg on Monday.

According to several hundred messages on Apple’s discussion boards, the Snow Leopard problem is triggered when users log in and out of an old Leopard guest account and then try to log back in to their regular account. One victim described the effect as follows:

“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, caused lockups, spinning beach balls, loud fans… and it was just getting worse. I am restoring Leopard (sans snow) as I write this.” (link)

The Sidekick disaster is of a different magnitude — which may be why Apple is willing to have the two discussed in the same news cycle.

On Saturday, Microsoft and T-Mobile warned Sidekick owners that any data not saved on their devices may have been lost due to a massive server failure in Microsoft’s Danger division. T-Mobile halted Sidekick sales and offered affected users a $100 rebate while it tried to recover the lost data.

In a note to clients issued Tuesday, Kaufman Bros.’s Shaw Wu suggested that as many as 1.5 million customers may now be “ripe for the picking” by Sidekick’s competitors.

“We believe RIM, Apple and Palm are well positioned to gain customers,” he writes. “Out of the three, we believe RIM may be best positioned as it is already at T-Mobile, has a physical keyboard that Sidekick users crave, and arguably has the industry’s best and most secure platform for messaging and e-mail communications.”

For entertaining — if somewhat biased — accounts of the problems plaguing Microsoft’s Danger division, I recommend Daniel Eran Dilger’s posts on AppleInsider and Roughly Drafted Magazine here, here and here.

[Follow Philip Elmer-DeWitt on Twitter @philiped]

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