Title says it pretty much and came to us from a client. The initial concern was that this had occurred due to an upgrade. There was plenty of evidence in the log tables however to show that the data had been deleted by a particular user in bulk.
The reason proved to be that an admin user removed a Location Type. There is a clear warning if you try to do this via the UI.
WARNING: Deleting this option will result in the loss of all location type records which use the option. This may mean the loss of a substantial amount of data, and the action cannot be undone. Do you want to continue?
Have asked about making the warning more specific in CRM-17273: Deleting Location Type should warn about loss of related contact data
Eventually it was identified that there is a way this can be done inadvertently when editing a single contact record in order to delete a single address record. This came up on this JIRA ticket.
In the scenario we investigated, it seems that the user was editing a single record, and in order to edit a single address entry they accidentally / inadvertently clicked on the 'spanner' icon to the right which opens up the Location Type page, and this gives access to the Delete button which it is concievable the person might click on thinking they are deleting the single Address, rather than the Location Type.
They do then get the big warning noted in Andrew's answer which we are suggesting could be improved in the JIRA ticket.
Note that the user requires 'administer CiviCRM' permission to get that link to edit location types.
It was possible to restore the data from the log_tables in this case.