Add one onto the early-'90s failure list: IBM Workplace OS. Rarely mentioned these days for some reason, but a hugely hyped and super-expensive complete failure.

From the Wikipedia page https://en.wikipedia.org/wiki/Workplace_OS: A University of California case study described the Workplace OS project as "one of the most significant operating systems software investments of all time" and "one of the largest operating system failures in modern times".