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Margaret Hamilton, credited with coining the term software engineering, standing with the code she designed for the Apollo 11.

Hamilton's work prevented an abort of the Apollo 11 moon landing: Three minutes before the Lunar lander reached the Moon's surface, several computer alarms were triggered. The computer was overloaded with incoming data, because the rendezvous radar system (not necessary for landing) updated an involuntary counter in the computer, which stole cycles from the computer. Due to its robust architecture, the computer was able to keep running; the Apollo onboard flight software was developed using an asynchronous executive so that higher priority jobs (important for landing) could interrupt lower priority jobs. Initially, the fault had been attributed to a faulty checklist and the radar being erroneously activated by the crew, but a 2005 re-analysis concluded that a hardware design error in the rendezvous radar provided the computer with faulty information even while in standby mode.

Due to an error in the checklist manual, the rendezvous radar switch was placed in the wrong position. This caused it to send erroneous signals to the computer. The result was that the computer was being asked to perform all of its normal functions for landing while receiving an extra load of spurious data which used up 15% of its time. The computer (or rather the software in it) was smart enough to recognize that it was being asked to perform more tasks than it should be performing. It then sent out an alarm, which meant to the astronaut, I'm overloaded with more tasks than I should be doing at this time and I'm going to keep only the more important tasks; i.e., the ones needed for landing ... Actually, the computer was programmed to do more than recognize error conditions. A complete set of recovery programs was incorporated into the software. The software's action, in this case, was to eliminate lower priority tasks and re-establish the more important ones ... If the computer hadn't recognized this problem and taken recovery action, I doubt if Apollo 11 would have been the successful moon landing it was.

—Margaret Hamilton, Letter to Datamation, March 1, 1971

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