Just to evangelize D a little and increase my code/crap ratio, let's pretend we develop a library in C++ that contains this class:

class SomeMetaVariables {
    public:
    std::string foo;
    bool bar;
};

// ... Somewhere in the client code:
SomeMetaVariables baz;
baz.foo = "foo";
baz.bar = true;

Our library is quite successful and many people are using SomeMetaVariables despite its obvious flaws. Now, say we get many requests for additional functionality, for example: "Make bar true only when foo is set to "foo" and other way arround." "Well, ok." - we say and commit this new version of SomeMetaVariables:

class SomeMetaVariables {
    std::string foo;
    bool bar;

    public:
    std::string getFoo() {
        return foo;
    }

    std::string setFoo(std::string newFoo) {
        foo = newFoo;
        bar = (foo == "foo");
        return foo;
    }

    bool getBar() {
        return bar;
    }

    bool setBar(bool newBar) {
        bar = newBar;
        foo = bar ? "foo" : "";
        return bar;
    }
};

We implemented the requested feature, but SomeMetaVariables' interface has changed... "But why are you mad clients? You asked for it!" - cries the C++ developer.

Let's suppose we have this class hierarchy:

It's a fairly specialized class hierarchy providing some functionality, let's say for the sake of this example that SomeClass provides a kind of player-game interaction interface and HierarchyWith defines it as a trigger based interaction (as in "a player does something - it triggers a response"). Our ExampleClass represents an entity that can be triggered by the player and that generates some response and in addition to that is further specialised to have some concrete functionality - let's say it's a lever that a player determined enough can pull to flood something with delicious magma. As we clearly can see ExampleClass is well-defined in terms of OOP, it's highly specialised and modular, but as an in-game entity it lacks a rather crucial functionality - it can't even be displayed on the screen. What do we do now?