Always display our custom 404 page

app/controllers/application_controller.rb

@@ -65,6 +65,9 @@ class ApplicationController < ActionController::Base
   filter_parameter_logging :password
 
   rescue_from ActionController::InvalidAuthenticityToken, :with => :invalid_authenticity_token
+  # FIXME: This doesn't work with Rails >= 3.0 anymore
+  # Possible workaround: https://github.com/rails/rails/issues/671#issuecomment-1780159
+  rescue_from ActionController::RoutingError, :with => proc{render_404}
 
   include Redmine::Search::Controller
   include Redmine::MenuManager::MenuController

app/helpers/application_helper.rb

@@ -436,8 +436,8 @@ module ApplicationHelper
       css << 'theme-' + theme.name
     end
 
-    css << 'controller-' + params[:controller]
+    css << 'controller-' + params[:controller] if params[:controller]
-    css << 'action-' + params[:action]
+    css << 'action-' + params[:action] if params[:action]
     css.join(' ')
   end
 

test/integration/application_test.rb

@@ -49,4 +49,13 @@ class ApplicationTest < ActionController::IntegrationTest
     assert_response 200
     assert_nil session[:user_id]
   end
+
+  def test_always_use_custom_404
+    get 'something_not_existing'
+    assert_response :not_found
+
+    assert_tag :tag => 'p',
+      :attributes => {:id => 'errorExplanation'},
+      :content => "The page you were trying to access doesn't exist or has been removed."
+  end
 end