Defect Leakage is natural. It’s common. If you build a software, bugs will be there. Similarly even if you Test a software, you cannot prove their absence. But we can do our best to eliminate maximum of them. Typically in most of the projects even if you complete functional testing (system testing) without any time & cost constraints, still UAT team (Business) manages to catch some more bugs. Any idea why? There has to be something missing in the system test coverage. What is it?