Abraham Wald and the Missing Bullet Holes
The statistician Abraham Wald is credited with telling the military to armour the parts of its bombers that showed no bullet holes. The advice sounds backwards until a reader asks which planes were never seen.

During the Second World War, analysts counted the holes in every bomber that returned from a mission and found that bullet holes were concentrated on the wings and fuselage, while the engines had far fewer. Military planners therefore proposed adding armour to the wings and fuselage, since these were the areas most often hit by enemy fire.
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