In WWII, bullet holes in planes had to be repaired. But the damage wasn't uniform - some areas got badly shot up, some areas were never damaged. So they added a

In WWII, bullet holes in planes had to be repaired. But the damage wasn't uniform - some areas got badly shot up, some areas were never damaged. So they added armour to the undamaged areas, because the planes that got shot there never came back. Similar idea from another angle: superficial design flaws obscure fundamental design flaws (Douglas Adams on the Sirius Corporation Complaints Depatment)