Zero. A defect like this absolutely should have been caught by quality control. Even if they only take some sample size of parts for detailed analysis from eve

Zero. A defect like this absolutely should have been caught by quality control. Even if they only take some sample size of parts for detailed analysis from every run, this would have been readily apparent. If it wasn't visible at time of manufacture (sometimes there can be a thin "skin" that covers underlying defects like this) this would have been evident from other inspection methods - X-ray analysis is commonly used to check for internal porosity or cracks in safety critical applications like this.