Well, for some reason it doesnt let me respond to the child comments :(
The problem (which should be obvious) is that with a/b real you cant construct an exhaustive input/output set. The test case can just prove the presence of a bug, but not its absence.
Another category of problems that you cant just test and have to prove is concurrency problems.
The problem (which should be obvious) is that with a/b real you cant construct an exhaustive input/output set. The test case can just prove the presence of a bug, but not its absence.
Another category of problems that you cant just test and have to prove is concurrency problems.
And so forth and so on.