If you google for "apollo 1201 or 1202 alarm" you'll find several good magazine-length articles on the design of the landing guidance computer systems.
The TLDR is: a switch misconfiguration was tasking the computer to process some rendezvous tracking information which was not needed. This exhausted a rather limited set of storage locations and triggered the alarm.
The computer was properly programmed to treat the condition as a secondary failure and continue with its primary task.
The thing that was remarkable to me was the control room engineer's response to the 1201 right before touchdown - they hadn't seen one of those, and it was not the same as the 1202's they had seen earlier.
In the videos I've seen you hear the audio - "1201 alarm" and within less than a second the 28-year-old (average age, just guessing) responds "1201 go". He just gave instant clearance to proceed past a malfunction a few hundred feet above the moon's surface, in real time. Talk about being in the zone.
The TLDR is: a switch misconfiguration was tasking the computer to process some rendezvous tracking information which was not needed. This exhausted a rather limited set of storage locations and triggered the alarm.
The computer was properly programmed to treat the condition as a secondary failure and continue with its primary task.
The thing that was remarkable to me was the control room engineer's response to the 1201 right before touchdown - they hadn't seen one of those, and it was not the same as the 1202's they had seen earlier.
In the videos I've seen you hear the audio - "1201 alarm" and within less than a second the 28-year-old (average age, just guessing) responds "1201 go". He just gave instant clearance to proceed past a malfunction a few hundred feet above the moon's surface, in real time. Talk about being in the zone.