XP

After allied planes encountered Nazi anti air fire in WW2, the allies naturally wanted to reinforce the planes. The planes were riddled with hundreds of holes surrounding a few key areas. Naturally the military wanted to reinforce those areas as they seemed like the areas most likely to be shot.

This was until one person by the name of Abraham Wald pointed out that this was the damage on planes that made it back.

Thus reinforcing those areas would have no positive effect and would make the planes unnecessarily slow. The correct decision would be to reinforce the areas that weren't damaged as they could be potential vulnerabilities.

This phenomenon is also known as survivorship bias that takes place when you only focus on the group that survived.