The bullet holes nobody saw saved the bombers.
In the Second World War the US military wanted to armour planes where returning aircraft had the most bullet holes. Statistician Abraham Wald objected: those planes had made it home despite the hits. The armour belonged where there were no holes, because planes hit there never came back. You only see the survivors.
Useful: For every success story, ask how many did the same thing and failed.
Source: en.wikipedia.org