While cracks are often considered the "worst" weld defect, the most dangerous ones are those that go undetected and are put into service. Many common discontinuities like lack of fusion, internal porosity, or even small surface cracks can be missed if quality control standards are inadequate or if inspections are timed improperly. For example, hydrogen-induced cracking can appear up to 48 hours after welding, and incorrect shielding gas for stainless steel, though visually appealing, can lead to costly premature corrosion or sensitization in service. Effective quality control, including qualified procedures, trained welders, and appropriate inspection timing and methods, is essential to prevent these hidden, catastrophic failures.