when I do:
# ifconfig wlan0 create wlandev ath0 wlanmode adhoc

server reboot with kernel panic:

Fatal trap 12: page fault while in kernel mode
cpuid = 1; apic id = 01
fault virtual address   = 0xffff
fault code              = supervisor read, page not present
instruction pointer     = 0x20:0xc0791612
stack pointer           = 0x28:0xd2f42ba4
frame pointer           = 0x28:0xd2f42bac
code segment            = base rx0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 0 (ath0 taskq)
trap number             = 12
panic: page fault
cpuid = 1
Uptime: 3m35s
Physical memory: 439 MB
Dumping 80 MB: 65 49 33panic: bufwrite: buffer is not busy???
cpuid = 1
 17 1

#0  doadump () at pcpu.h:246
246     pcpu.h: No such file or directory.
        in pcpu.h
(kgdb) #0  doadump () at pcpu.h:246
#1  0xc06b3497 in boot (howto=260) at /usr/src/sys/kern/kern_shutdown.c:416
#2  0xc06b3789 in panic (fmt=Variable "fmt" is not available.
) at /usr/src/sys/kern/kern_shutdown.c:579
#3  0xc08b63bc in trap_fatal (frame=0xd2f42b64, eva=65535)
    at /usr/src/sys/i386/i386/trap.c:938
#4  0xc08b6620 in trap_pfault (frame=0xd2f42b64, usermode=0, eva=65535)
    at /usr/src/sys/i386/i386/trap.c:851
#5  0xc08b6f39 in trap (frame=0xd2f42b64) at /usr/src/sys/i386/i386/trap.c:533
#6  0xc0899b3b in calltrap () at /usr/src/sys/i386/i386/exception.s:165
#7  0xc0791612 in ieee80211_getcapinfo (vap=0xc2d5f000, chan=0xffff)
    at /usr/src/sys/net80211/ieee80211_output.c:1836
#8  0xc0793d17 in ieee80211_beacon_construct (m=0xc2edca00,
    frm=0xc2f0516e "", bo=0xc2d5f884, ni=0xc2ceb000)
    at /usr/src/sys/net80211/ieee80211_output.c:2559
#9  0xc07946db in ieee80211_beacon_alloc (ni=0xc2ceb000, bo=0xc2d5f884)
    at /usr/src/sys/net80211/ieee80211_output.c:2756
#10 0xc050eaea in ath_newstate (vap=0xc2d5f000, nstate=IEEE80211_S_RUN,
    arg=-1) at /usr/src/sys/dev/ath/if_ath.c:2641
#11 0xc0798db1 in ieee80211_newstate_cb (xvap=0xc2d5f000, npending=3)
    at /usr/src/sys/net80211/ieee80211_proto.c:1654
#12 0xc06ebf92 in taskqueue_run (queue=0xc2a84c80)
    at /usr/src/sys/kern/subr_taskqueue.c:239
#13 0xc06ec19d in taskqueue_thread_loop (arg=0xc2ada074)
    at /usr/src/sys/kern/subr_taskqueue.c:360
#14 0xc068a331 in fork_exit (callout=0xc06ec0e0 <taskqueue_thread_loop>,
    arg=0xc2ada074, frame=0xd2f42d38) at /usr/src/sys/kern/kern_fork.c:843
#15 0xc0899bb0 in fork_trampoline () at /usr/src/sys/i386/i386/exception.s:270
(kgdb)

Fix: 

Don't know.
How-To-Repeat: 
	ifconfig wlan0 create wlandev ath0 wlanmode adhoc