According to multiple topics on the Apple Support discussion boards, the problem can occur when a user logs into their Mac's Guest account -- whether by accident or on purpose -- and then tries to log back into their regular account.
In some cases, users have reported finding their regular account empty of data, as though it were a brand new account.
Speculation is that something makes Snow Leopard treat the regular account like a Guest account, from which by default all data is deleted upon logout. Further speculation is that the problem occurs when the Guest account was already enabled in Leopard before being upgraded to Snow Leopard.
The problem started being reported within days after Snow Leopard's shipping, but the discussions are all still marked "not answered"
The only solution for getting the deleted data back is to restore it from a backup.
An article on MacFixIt suggests disabling the Guest account entirely, or at least disabling it and then re-enabling it so that it's a native Snow Leopard account. Another suggestion for those who need a temporary account is to create a new Standard account and use Parental Controls to restrict it.
Snow Leopard bug can delete account data
A bug has been reported in Apple's new Snow Leopard version of OS X that can result in the loss of an entire user account's data. The glitch seems to be triggered by using a Guest account and then trying to log back into a regular account.
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