To my shame, someone discovered a way to crash out of the xscreensaver lock dialog.
The proximate cause was that an internal consistency check failed because I meant to be checking "am I running on the signal stack?" but I was checking "is the SIGCHLD signal currently inhibited?" instead. And that condition was triggered by hot-swapping monitors while the display was locked.
But wait, why does your code that prompts for passwords need to have signal handlers and manually and explicitly respond to hardware reconfiguration? Because fuck you that's why.
Blah blah X11, complexity, privilege separation, doomed, get a Mac.