This image tracks the bullet/cannon shell holes on Allied plans that encountered German anti-aircraft fire and German fighters in World War II.
At first, the military wanted to reinforce tosses areas hit, because obviously that’s where the ground crew observed most damage on returning planes. This was until Hungarian born Abraham Wald pointed out that this was damage to the planes that had managed to get back to home.
Wald went on to say that the Allies should armour the areas where there are no dots at all, because those are the places where the planes won’t survive when hit.
This phenomenon is called survival/survivorship bias, a logical error where people focus on things that when you should really be looking at the those that didn’t.