As a data-point, this happened again while XDM was running things.
I was able to partially recover - in that after killing both X
servers, XDM restarted one on vt2 that seemed fully functional, but
when I then tried to switch to vt5 the machine hung hard and I
eventually had to hit the reset button.  Damage was minimal thanks to
hardware mirror'ing, but I think we'll stop trying to share a terminal
;-)

In case it matters, the machine is:

NetBSD void.crufty.net 1.6_STABLE NetBSD 1.6_STABLE (CRUFTY) #0: Sun Nov 10 16:37:28 PST 2002     root@void.crufty.net:/j/obj/sys/compile/CRUFTY i386


--sjg