Wait, are you sure indentation was the real problem there? What you're describing sounds more like a scope issue, where the print was using a variable that got reset inside the loop, so moving it up just changed the order things ran. I've seen new folks blame indentation when really their logic was just out of order, and that's a way harder habit to unlearn than a tab vs space thing.

Dana's secret ingredient was probably just being the cheaper option.

Man that is rough, 11 hours on a 300 HP firetube and it ends up being a 4 dollar gasket. I have been there way too many times, chasing water loss on a big boiler and everything looks dry because the leak is flashing off before it ever hits the floor. The worst ones for me are the handhole gaskets and the rear door, you can hear a faint hiss if you get close but by the time you grab a light it is gone. I started using a bright flashlight and looking for that little white mineral trail or rust streak instead of actual water, and sometimes a mirror on a stick helps you spot the wet spot on the tube sheet before it dries.