I've had shadows turned on for a while, but not now.
This morning my computer crashed hard a couple of times. It seems to be signaled by flashing of a window on the other monitor.
I was lucky; I was able to sign on for long enough to turn off renderdeferred, after which the problem went away... but of course, so did the shadows.
I think I'll submit two JIRA entries, one being to allow setting of debug options before signing on.