Re: Many years ago...
I've been a part of a SCADA technology development team since 2003; I'm mostly into communication protocols, databases, and archiving (historian).
Our systems are used in industry, energy control (electricity, gas), chemical factories, and transport (railroads). They are often built as redundant 2 or 3-node systems, so they are meant to go 24/7/365. The planned outages during patching are 0, the outages during upgrades are counted in minutes (see https://d2000.ipesoft.com/blog/dispatch-control-system-upgrade-scada).
We (developers & application specialists) acknowledge we are human and we make mistakes. When we do, we analyze them, correct them and learn from them. Btw, we are using Ada language that helps us to make fewer mistakes and find them faster (by features like strict typing, runtime checks, and high readability).
Also our customers are technically-oriented, so they acknowledge the new version may have issues that escaped testing, but as long as we can correct them within hours since being reported, they are satisfied.