I have gotten this several more times and I think I know what is causing it.
I had the Project toolbar undocked and resized to be narrow and tall.
I moved this toolbar to the left side of the main window and when I 
left-clicked to dock it there, the click happened to be inside the
Restore Environment to Normal Mode button (the environment was forced into
Debug mode when I started moving the Project tool bar).  I believe this
is what triggers the precondition violation.