The Therac-25 (https://en.wikipedia.org/wiki/Therac-25) didn't have a complex GUI. However, I hope that those affected by that bug did get their money back (and then some). Not that money can really help you if you get exposed to fatal levels of radiation, but...
GUIs do nothing more complex than many things that have to work right, so do. It is only tolerance for crappy software that allows it to be foisted on us.