This photo is a picture of a US production sailboat seacock assembly as shipped from the factory. While this builder no longer uses gate valves they still, to this day, install seacocks using this same method.
This owner simply wanted to remove the gate valve and replace it. When a slight pressure was applied, not even enough to break the threads free, or spin the thru-hull in the hull, the thru-hull snapped off and he was left holding the valve assembly in his hands. WOW!!!!!
This owner installed through bolted "proper" flanged seacocks. This could happen to anyone with this type of installation and this owner is just lucky it did not happen while off sailing.
If you look close you can still see the broken thru-hull still in the bronze elbow..