" ... Let's be honest. Your tests mostly follow the “happy path”. Sure, on occasion you remember to test “the failure case”—the caller passed in null or a negative integer as an argument—mostly because you just got done writing that check and wouldn't it be a waste not to write a test to show how clever you were? Never mind that null or a negative argument is an assertable precondition that could never happen in production anyways. ... " The way I've felt for years. Roland Backhouse had an even more damning critique of testing as a method for catching bugs in one of his books.