As far as I can tell, this change has broken error handling so the failsafe_response now always kicks in: - ActionController::Base.process_with_exception(request, response, exception).out(output) + ActionController.process_with_exception(request, response, exception).out(output) the process_with_exception method is indeed a class method on the ActionController::Base class, not on the ActionController module. Here''s the underlying error, which is being suppressed by failsafe_response: Error calling Dispatcher.dispatch #<NoMethodError: undefined method `process_with_exception'' for ActionController:Module> /Users/lars/Sites/isabont/config/../vendor/rails/railties/lib/ dispatcher.rb:42:in `dispatch'' It''s not clear to me what the change was trying to accomplish, and now that dev.rubyonrails.org is down, it''s hard to find out (commit log says it''s closing #4625). /Lars