&gt; &gt;considering that comprehensive testing across the entire space of possible inputs for a unit is infeasible in most cases
&gt; 
&gt; Fortunately, it is also not necessary, as numerous studies have found.<p>If you know something about how some function or module works and you check the right corner cases, the only way a bug could be hiding in some of the other input combinations is if it were deliberately planted in the form of extra code, like &quot;if the inputs are specifically x, y and z, then misbehave such and such&quot;.<p>For instance if we know that a 32 bit adder is made up of bunch of full adders, there are certain bit patterns we can use which tell us that the adders are working properly individually, and that they are hooked up together right. We don&#x27;t have to test every A + B = C triplet, blackbox style.