In the second World War, bombers returning from missions over Germany were carefully examined for battle damage and the locations of bullet holes mapped. Armour plating is heavy and cuts into the planes' useful range and load---where should it be placed? Abraham Wald suggested putting armour plate over places where no bullet hole had ever been observed, because those were the vulnerable spots. Planes shot in those locations never returned to be counted.
It's called the problem of non-ignorable non-response.
It's called the problem of non-ignorable non-response.