I’m not sure how it got there on the heads you’re working on, but I’ve seen Edelbrock SBM heads that have the exhaust bolt hole broken thru into the exhaust port right ootb.
I don’t think I’ve ever ran into that yet out of the box. All I can add is we all screw up. I felt like crap earlier this year when a customer found one of my pushrod tubes leaking. He quickly fixed it but it still bothers me. I have made a couple changes since so hopefully it never happens again.