There is a critical bug in CiviCRM 1.9 that causes data loss.
Steps for reproducing:
1. Find a contact with an existing location.
2. Click the "Delete" link next to the location. This will take you to a confirmation screen.
3. Click "Delete Location" to remove this location.
4. When completed it will take you back to the contact's summary screen
5. Press the 'back' button in your browser.
6. You will again be greeted with the confirmation screen.
7. Click "Delete Location" again (for an already deleted location)
This will cause CiviCRM to delete ALL rows in the address and location tables.
Running PHP 5, MySQL 4.1, Apache 1.3, Drupal 5.7.
Please let me know if this is reproducible on other systems, and if there is a possible fix.