In 2000, I read about CI/CD, but there weren’t tools yet (CruiseControl came out in 2001). So, we implemented CI by having a physical card that had to be on your desk in order to commit code. Then, you had to go over to our build (desktop) machine, get your code, run the build/test scripts (and fix if you broke them).
We eventually automated it all, but just that step saved a lot of “works on my machine” bugs from breaking our builds.
