Sync errors are not generally alarming - most are just Outlook catching up to changes in the mailbox. See: http://support.microsoft.com/kb/2606009. "In most cases, these errors are logged in the Sync Issues folder when the items no longer exist in the Exchange mailbox. In these cases, the error can be safely ignored." If the error is repeatedly the same event, it may be a corrupted item and it's best to delete it.