Commanded by Alan Shepard, the only original Mercury astronaut to make it to the Moon on an Apollo mission, Apollo 14 was a reflight of Apollo 13′s abandoned lunar landing plan.

However, less than four hours before the scheduled landing, controllers noticed that according to the indications on their consoles in Mission Control, the LM’s Abort pushbutton appeared to have been pressed. When asked via radio, Shepard confirmed that no one on board Antares had pressed the Abort button—which meant there was a short-circuit or other electrical issue somewhere inside the LM’s complicated guts.

This was potentially a mission-ending problem: if the button was pressed and the engine was firing, the LM would immediately begin its abort procedure as soon as the lunar descent started, making a landing impossible.

Under hard time pressure, the ground had to quickly figure out what was wrong and devise a workaround. What they came up with was the most brilliant computer hack of the entire Apollo program, and possibly in the entire history of electronic computing

all the software for the flight to the Moon and back had to fit in 36K words (15 bits long, plus 1 bit for parity) of read-only core rope memory

AGC had a trivial 2k words of RAM—necessary for the operating system, process management, recovery, and global variables for all mission phases. That’s it.

Up to eight Jobs could run in the LM’s AGC (seven in the CSM’s AGC),

The crew, superbly trained in all aspects of the mission, demonstrated their superior tapping skills and the Abort bit cleared itself.

The failure was unpredictable, and true to Murphy, would likely recur at the worst possible time.

idea that a single errant switch could derail a lunar landing attempt was unacceptable. After the mission, a new variable in the AGC code was introduced that allowed the crew to “mask out” (that is, to ignore) the Abort and Abort Stage pushbuttons. The scenario assumed that a failing switch would be recognized well before the descent began, and commands could be entered in time to prevent an inadvertent abort.