Sounds like you were probably using gnome-screensaver or some of the many other poorly written alternatives like cinnamon that do this. I don't believe there is any way for xscreensaver to unlock the desktop even if it does crash
Incorrect. It’s a limitation of X11 that if the screensaver daemon crashes, including xscreensaver, the desktop will be unlocked. See the JWZ links that are posted in this thread.