The software that flew Apollo to the Moon was physically woven by hand. Programs were stored as core rope memory, with workers passing a wire-threaded needle through tiny magnetic rings; a wire through a core meant one, around it meant zero.

Because the code could not be altered without rewiring, the design had to be frozen weeks ahead, which engineers treated as a built-in guarantee against last-minute bugs.

Source: Core memory weavers and Navajo women made the Apollo missions possible (Science News)