Processed a strict pressure test on a C.S ball valve with DN300,600#, during the test there's no leakage. While 6 months later on site customer find a crack with 20mm length.
Compere to a crack in a car front window: an impact (stone or grit flying) can give an invisible, or almost invisible material damage, and because of internal stress and variation in stress (pressure and temperature) widen to be a crack across the complete window.
A crack in a valve body (or a ball valve) , without any other damage in the system, you wold strongly suspect to have started from some material imperfection and be widened from stress.
Metallurgic microscopic sample would probably  be able to confirm the material fault.
A particular material imperfection may well have passed all previous inspections, even thorough different first-class types especially if they occur on 'non suspect' sections of the valve(cases seen personally).

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