Jumping between Guest account and Normal one wipes out data
Apple had released the first batch of updates for its new Mac OS X 10.6 Snow Leopard last month. Apple Discussion boards had several users who complained about a recent bug related to Guest User account led to data deletion from the user's Mac. The settings are being reset and data is reported to be wiped out.
The bug appears when Snow Leopard user logs into Guest account, accidentally or on purpose, then logs off and logs into normal account. Having done that, this normal account may be found as fully wiped and reset as fresh one. Also, the home directory would exist under "/Users/username" but would show no data. It's being speculated that users with guest account enabled in Leopard and upgraded to Snow Leopard faced this problem.
However, the number of users impacted by this bug is uncertain. Performing regular data backups are always advisable and recommended here. As per MacFixIt's suggestion, the Guest Account causing menace can be disabled entirely or disabled and then enabled again as Snow Leopard's account. In either of the cases, users can always create a separate user account with respective controls set in it.
These topics at the Apple Discussion Boards are left unanswered. It is not sure if the bug affects large number of users or only select few. At the most, Apple will release software update for fixing the bug.