A few days ago I noted a quick but very usefull tip on how to handle the situation when Front Controller is unable to dispatch the request, i.e. how to display 404 error page instead of Zend_Controller_Dispatcher_Exception with message "Invalid controller specified". And it looks like the common solution for this is to create a front controller plugin. The plugin detects whether the request is dispatchable and changes module/controller/action to appropriate action that will handle the request.

His solution uses a NoRoute controller to check and see if there is a valid controller for the requested action. If not, it routes them to a custom controller with a nicer error message. For those that want a bit more low level type of solution, he also includes how to modify the framework itself to perform the same action.