The main aim of testing is to execute some test cases, manually or through automation with the aim of finding the unknown errors/bugs in the given software application. So many times, I have observed, testers calling a particular Test Case as good test case which always passes. Is it right? No! a good Test Case is the one which has a very high probability to fail. Let me give a example of my previous project. It was a GUI application.  If the user clicks onSubmit button in the given form, a new page was presented to the user with the modified URL at the address header of the browser window. 

                               Everything looks absolutely fine. But if you cut and paste the modified URL to the new browser window and press Enter key, this fails to bring the same page. According to me this is a good test case which we simply don’t think. Hence, the design of a Test Case should be such that so it can uncover the unknown errors.  That’s why I always give lots of importance to the testers as they think Out of Box. The intention of testing is not to show that the given application is free of bugs. But to say that defects are present in the application, so better fix them before you ship the application to the client. 

                                Please note down that if the bugs have slipped in the testing phase and discovered either by the end users or client. The cost to fix them during the maintenance phase is 50-100 times of the cost fixed during the development phase.