Half an operating system: The triumph and tragedy of OS/2
OS/2 had many flaws but its multitasking was unseen on a PC at the time. I remember formatting a floppy while running two 16-bit Windows sessions (which were communicating with each other) and multiple DOS windows, thinking I was in the future.Even Windows 95 was limited by many system calls being funneled through single threaded BIOS or DOS 16-bit land.