during world war 2, the US military would inspect returning bomber planes for damage. the planes that successfully returned were riddled with bullet holes and pieces blown off from enemy fire. the military would then patch up the holes and instruct future planes to be extra reinforced in the areas of the holes.
and then abraham wald, a mathematician, came along.
yes, it would make sense to add extra shielding to the places were the holes were, but, he argued, the holes were not an indication of a problem. if anything, they knew that planes with these holes were capable of returning.
he concluded that it was the areas were there were no holes that were the real problem, as those planes that had this damage never returned.
the military took his advice and instead focused on the areas where there were no holes, improving the safe return of pilots on future missions.