You misunderstand interfaces and factories ...
You stated the following:
"But the programmer has to know what object is being returned and write code apropriately. Unless every method in every class exists in every other class. If any of the possible child classes has a public function that doesn't exist in the others the programmer has to know not to call it."
In a statically typed language like Java what you wrote is simply not true. A factory should always return an interface. Therefore any Object that you get back from the factory by definition implements all of the methods in the interface. Therefore the only, methods that you can call are the interface methods. The programmer doesn't have to know anything more then the interface. From his perspective the only methods that exist are those that the interface declares and any object that he gets backs is guaranteed to implements them therefore there are no decisions.
In a dynamically typed like language like Smalltalk what you said is theoretically possible but from experience almost never happens. Factories return classes that implicitly implement some interface and those are the methods that are used.
In fact, in a dynamically typed language you never whether an Object implements a method, it is only checked at runtime. In reality, that is not an issue. You know what type of Object you are supposed to be getting.
