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?

Testing in programming is pretty important, no doubts about it, too bad that some languages make it difficult to test the code easly. For instance C++ is pretty limited in terms of DbC and unit testing. All it ever supplied to programmers was a little =assert()= macro that stopped the program providing minimal info about the error and the place it was "caught" in. I mean there are exceptions in C++ and they are pretty usefull, but still it's not enough and requires lots of effort in lerning and implementing correctly. One can still say there are libraries such as CppUnit for those purposes, but we all know how it goes - the more I need to do to get some minor advantage the less I'm interrested in it, and unit testing is so underestimated it's not considered an advantage to have a working piece of code anymore, how about that? And speaking of DbC in C++... It is possible... bam (A sound of a thousand readers going "whaaa" this instant.)