The latest version of QCad (qcad-18.104.22.168-1-prof.linux.x86) just quit without saving modified files. After about an hour of working (and several saves), I quit via the window manager, and didn't receive a prompt to save any unchanged files. Restarting QCad and loading up the files I was working confirmed that I had lost work. Strangely, I couldn't get QCad to do it again.
If I ever find a way to repeatedly cause this, I'll share it here.