What to do about "Operation failed: Update Indices DB Error: unknown error Debug" after upgrading to 4.7.25
You'll have to work on your database directly. The following works for me - although the Status screen still shows the error until the Update Indices button is clicked again after the repair has been done (I presume it simply verifies the fix):

USE your_civicrm_db;
SHOW CREATE TABLE civicrm_case_contact;
SET foreign_key_checks = 0;
DROP INDEX `UI_case_contact_id` on civicrm_case_contact;
CREATE UNIQUE INDEX `UI_case_contact_id` ON civicrm_case_contact (case_id, contact_id);
SET foreign_key_checks = 1;


If you do this via PhpMyAdmin (or similar), you can skip USE your_civicrm_db; since you'll have already navigated there.