The Juno II rocket launch in 1959 spectacularly failed when the rocket did a U-turn after launch. The cause was two diodes touching due to vibration. After that, rocket components were often potted in plastic to prevent similar problems.

Video of launch: https://www.youtube.com/watch?v=885as-VE50g&t=1446s Discussion of diodes: https://youtu.be/885as-VE50g?t=1688

Same for plenty of vehicles, early ECUs were often potted and the ones you see today sometimes still have a resin coating on the component side to harden the device against vibration.

How does one knew at that time that is the issue … must be a great detective work!

62 years ago. Just letting that sink in. (Thanks for the video links!)

My dad said he referred to those potted assemblies as “integrated circuits” when working on TV bandwidth compression for the phone company in early 1960s australia.