A fairly poor country was in the middle of a war. Being on the lower-end of the economic scale, they didn't have enough money for fancy armor or weapons. They typically went into battle armed with nothing more than regular clothes, cloth helmets, and an assortment of older-model firearms.
A few months into the war, the ruler of the country decided that an investment in new Army helmets would probably be a wise choice (considering the large number of head injuries caused by the thin cloth ones). After spending nearly all the money the country could scrape together on new steel helmets, they were shocked to see the number of head injuries go up.
How could it be possible that steel helmets, assuming the helmets fit perfectly, caused more injuries than cloth?