Four minutes before the moon landing, the Apollo Guidance Computer began throwing 1202 and 1201 alarms - indicating that it wasn't completing its processing loop in time.
Here's the audio from the flight director and guidance loops, as they troubleshooted the problem in real time, clearing Apollo 11 to land.
tl;dr: Some misconfigured switches meant that rendezvous radar jobs kept being erroneously scheduled, which used up available memory. Whenever there was no more memory available, the alarm would fire and the computer was rebooted. It would immediately restart (resuming from near where they left off) all the high priority jobs (while not restarting the radar jobs), meaning that it could still land safely.
Jack Garman had hand-written down all the trouble codes on a "cheat sheet". He's the one who called "go". I am not even sure if the MIT software team knew what 1201 and 1202 were.
The AGC was one of the first to use priority multitasking. The 1201 and 1201 faults were indication that lower priority tasks were getting yanked.
"there were no specs, we made it up" "if you screwed up you got a stack of papers that was 2 feet thick" "the overall memory for the Apollo guidance computer was 72 kilobytes"
Clicked into the first one and got a thing about the insulation between the tanks - the 3rd video in the playlist was about them hiring surfboard makers to do the honeycomb insulation between the fuel tanks, since they had more experience working with the materials in the heat conditions.
Just another kind of "the space program was amazing" thing - the sheer diversity of people who got hired on to contribute is continually surprising. An amazing tribute to the pragmatism of NASA under the conditions.
It actually makes sense given the economics of the situation: time invested in high availability is a poorer use of resources than, say, adding features for most web sites.
We can make web sites that stay up, always. It's just that it costs a lot more and / or they do less.
Seeing as the computers were spitting error numbers instead of error messages, I am guessing that wasn't an option at that time with the hardware they had.
Four minutes before the moon landing, the Apollo Guidance Computer began throwing 1202 and 1201 alarms - indicating that it wasn't completing its processing loop in time.
Here's the audio from the flight director and guidance loops, as they troubleshooted the problem in real time, clearing Apollo 11 to land.
http://forum.nasaspaceflight.com/index.php?topic=35230.0