I did a 'shutdown -r' (from inside X) on a 1.5.2 machine; here's the 
resulting traceback:

#0  0x100 in ?? ()
#1  0xc02cb8af in cpu_reboot ()
#2  0xc018712d in panic ()
#3  0xc02d35fd in trap ()
#4  0xc0100f0d in calltrap ()
#5  0xc02184a6 in in6_delmulti ()
#6  0xc0223a31 in ip6_freemoptions ()
#7  0xc021b0e1 in in6_pcbdetach ()
#8  0xc0231522 in udp6_detach ()
#9  0xc02312c2 in udp6_usrreq ()
#10 0xc0194be5 in soclose ()
#11 0xc018aa62 in soo_close ()
#12 0xc0175344 in closef ()
#13 0xc017511e in fdfree ()
#14 0xc017672a in exit1 ()
#15 0xc0176630 in sys_exit ()
#16 0xc02d3d10 in syscall ()
#17 0xc0100fd1 in syscall1 ()
can not access 0xbfbfd434, invalid translation (invalid PDE)
can not access 0xbfbfd434, invalid translation (invalid PDE)
Cannot access memory at address 0xbfbfd434.

I don't think I was running any v6 applications.

		--Steve Bellovin, http://www.research.att.com/~smb
		Full text of "Firewalls" book now at http://www.wilyhacker.com