There was a lovely documentary I'm trying to chase down about John 'Jack' Garman had to "invent" a "a priority-scheduled multiprogramming operating system". This may have been related to the lander module though. The story was that when you were landing the lander, you better give priority to guidance because other things, like the temperature in the cabin for the next 15 seconds, didn't really matter if you crashed and burned. On the first shot they overloaded the computer and alarms started happening because some subroutines were not getting executed. There were too many loaded, but thanks to the priority concept, which Garman foresaw and built in because he thought it was a good idea, the low priority routines didn't bog down the higher priority landing routine.

Watching the documentary at the time, I was struck about how it was like doing a major refactoring on the code without telling management and almost getting fired because you were late on what you were supposed to be working on. In this case however, the refactoring came to light when reason for the alarms was investigated. (And management was still pissed! :-)

Some links:

No, a “checklist error” did not almost derail the first moon landing

TALES FROM THE LUNAR MODULE GUIDANCE COMPUTER

How They Built it: The Software of Apollo 11

NASA Johnson Space Center Oral History Project Edited Oral History Transcript

Brief history of NASA´s Apollo Program Excerpt: Five minutes into the descent burn, and 1,800 m above the surface of the Moon, the LM navigation and guidance computer produced the first of several unexpected "1202" and "1201" program alarms. The computer engineer at Mission Control Center in Houston, Jack Garman, told guidance officer Steve Bales it was safe to continue the descent. These alarms were indications of " executive overflows" , meaning the guidance computer could not complete all of its tasks in real time and had to postpone some of them.

Recalling the 'Giant Leap' Excerpt: We knew what that was and that it should not be happening. But we had designed a system that tried hard to recover from any overload conditions. So I remember hearing [NASA computer engineer] Jack Garman shouting, "Go, go!" And on they went. Then we listened as Neil flew the LEM on and on trying to find a good spot to touchdown. Our new worry was lack of fuel remaining. But finally we heard the contact notice and then, "The Eagle has landed."

Jack Garman Interview

EDIT: Maybe this was the documentary: Apollo 11: The Untold Story (2006)

Cast: John R. Garman ...

Himself - Apollo 11 Computer Engineer (as Jack Garman)

(Among others).

Update: A Hacker From South Africa Just Rescued the First NASA Computer in Space