Before the introduction of tin helmets during the First World War, soldiers only had cloth hats to wear. The strange thing was that after the introduction of tin hats, the number of injuries to the head increased dramatically. Needless to say, this was counter-intuitive. The new helmets were designed precisely to avoid or limit such injuries.
Of course, people were comparing apples with oranges, namely statistics on head injuries before and after the introduction of the new helmets. In fact, what they should have done, and effectively did after they realized their mistake, was to include in the statistics, not only the injuries, but also the fatalities. After the introduction of the new helmets, the number of fatalities dropped dramatically, but the number of injuries went up because the tin helmet was saving soldiers’ lives, but the soldiers were still injured.