The modern Combat Helmet, World War 1 onwards.
as the story goes, soldiers in the trenches were experiencing a large number of head injuries, so they were outfitted with helmets... this resulted in an Increase in the number of head injuries.
it was supposedly discovered that the number of head injuries increased because the number of fatal head inuries dramatically decreased thus saving countless lives from shrapnel and splintering material as a result of bomb and artillary blasts etc.
simple, effective and could be produced in large numbers
__________________
|