As mentioned by the Zend Developer Zone in this post, Aleksey Zapparov has posted a quick tutorial on how to deal with a missing action and controller in your Zend Framework application.

You can easily make them call indexController::noRouteAction() on unexistence controllers, but on unexistence actions they'll throw you an exception. So there is a way to change this - modify sources.

His code basicly watches for the exceptions that would be thrown from a missing action and handles then inside a try/catch. If things fail, the user gets shuttled off to the Error404 controller that dumps out the contents of the controller class.