An excellent example of focusing on the right thing:
II. During World War II, the Allies mapped the bullet holes in the planes hit by the Nazis and tried to strengthen areas where enemy artillery was heavily damaged.
Their goal was to rebuild and strengthen more red dotted (or more bullets hit) areas of the aircraft. Theoretically, this was a logical inference. After all, these were the most affected areas.
But Abraham Wald, a mathematician, came to a different conclusion: the red dots only represented damage to aircraft that could return home.
The areas that really need to be strengthened were where the shells did not hit. Because these were places where the plane could not survive when it was hit.
This phenomenon is called survival deviation. It is a mistake that you only focus on surviving examples.