Hi Marc - and there's more history for you to track down in the LM, related to Apollo 14.
About three hours before Apollo 14 was due to separate from the Command and Service Module (CSM) to start the lunar descent, Ground Control noticed a signal indicating that the Abort Button in the Lunar Module (LM) cockpit appeared to have been pressed.
The abort button was there in case something went badly wrong during the landing phase. With one push the Apollo 14 Astronauts (Shepard and Mitchell) could command a quick return to a safe orbit. At this point in the mission the spurious signal had no effect, but if it happened during the descent, the LM Ascent Engine would fire and take the Ascent Stage back up to lunar orbit before anyone could do anything about it. The astronauts would be safe, but the mission would be ruined.
As the Powered Descent Initiation (PDI) burn time approached, Mission Control noticed that amid the hundreds of bytes of data, a bit had been set tot 1 that should have been a 0. It was the Abort Bit, indicating that somehow, the LM Guidance Computer (LGC) had received an indication to abort the landing. The erroneous bit was harmless at this point, because the LGC in the LM was not running the descent program, so it was not looking for an abort input.
But once PDI was initiated and the braking phase program (P63) began, a faulty Abort Button could instantaneously lead to an uncommanded abort (P70), incorrectly aborting the landing - at best a massive disappointment, at worst a dangerous surprise.
To fix the problem, and about 19 minutes before loss of signal, Houston asked Mitchell to press the problematic ABORT button, and then the STOP button. This caused the faulty indication to go away. Houston then promised they would have further information once Antares reappeared from behind the Moon. About forty-five minutes later, Antares (LM) reappeared and everything looked good. But half an hour later, the Abort Bit set again. A little more than two hours remained before the critical PDI burn.
Ed Mitchell guessed that the intermittent fault may be the result of some loose solder debris in the switch assembly, floating around in zero-gravity, causing an intermittent electrical contact in the switch. (Subsequently, several similar switches were later x-rayed and found to have similar problems).
The workaround (by Don Eyles, the 27-year-old computer programmer who wrote the program) simply changed a few registers, first to fool the Abort Monitor into thinking that an Abort was already in progress, and then to clean up afterward so that the landing could continue unaffected. The amended procedure was radioed up to the LM and manually input by the astronauts using the on-board DSKY.
Although the ground controllers could have uploaded the procedure automatically into the LM AGC via the digital telemetry link, a decision was made to read the complex series of commands out aloud, up to the astronauts who then wrote them down and keyed the data in by hand.
I found this YouTube video playing the Apollo 14 landing from PDI to Touchdown absolutely fascinating.
https://www.youtube.com/watch?v=oZZe-xXx9_o
I have also attached some photos of the “Abort” and “Abort Stage” Buttons on the LM as seen during the NASA Exhibition at the Qld Museum, sans the yellow protective guards that protect these switches from accidental touches.
Cheers
Dennis
EDIT: Another YouTube video.
https://www.youtube.com/watch?v=wSSmNUl9Snw