If it returns INPUT, and there is no result defined for that (and no general result), you'll get an error page that says something like
No result defined for type INPUT and action <action name>.
You can define a general page that will always be shown for 'INPUT', but the better solution is that if you have a validate() method, it's usually a good idea to define an INPUT result to go back. Even if you don't explicitly do any validation, things like a non-integer value being passed to an integer field can cause that result, so it's a good idea to define it.