Index: app/controllers/auth_sources_controller.rb
===================================================================
--- app/controllers/auth_sources_controller.rb	(revision 17180)
+++ app/controllers/auth_sources_controller.rb	(working copy)
@@ -68,6 +68,8 @@
     unless @auth_source.users.exists?
       @auth_source.destroy
       flash[:notice] = l(:notice_successful_delete)
+    else
+      flash[:error] = l(:error_can_not_delete_auth_source)
     end
     redirect_to auth_sources_path
   end
Index: config/locales/en.yml
===================================================================
--- config/locales/en.yml	(revision 17180)
+++ config/locales/en.yml	(working copy)
@@ -221,6 +221,7 @@
   error_cannot_reassign_time_entries_to_an_issue_about_to_be_deleted: "Spent time cannot be reassigned to an issue that is about to be deleted"
   warning_fields_cleared_on_bulk_edit: "Changes will result in the automatic deletion of values from one or more fields on the selected objects"
   error_exceeds_maximum_hours_per_day: "Cannot log more than %{max_hours} hours on the same day (%{logged_hours} hours have already been logged)"
+  error_can_not_delete_auth_source: "This authentication mode is in use and cannot be deleted."
 
   mail_subject_lost_password: "Your %{value} password"
   mail_body_lost_password: 'To change your password, click on the following link:'
Index: test/functional/auth_sources_controller_test.rb
===================================================================
--- test/functional/auth_sources_controller_test.rb	(revision 17180)
+++ test/functional/auth_sources_controller_test.rb	(working copy)
@@ -159,6 +159,7 @@
           :id => 1
         }
       assert_redirected_to '/auth_sources'
+      assert_equal I18n.t(:error_can_not_delete_auth_source), flash[:error]
     end
   end