panic: blockable sleep lock (sleep mutex) process lock

I had this panic on boot before, now it happens on every shutdown :)
This is an SMP i386 box running -current built 20030427.
(Note: it's not even a reboot shutdown)

Is the in-kernel debugger broken or is it just not possible
to get there in some panic cases?

darkstar# shutdown now
Shutdown NOW!
shutdown: [pid 70995]
darkstar#

*** FINAL System shutdown message from root_at_darkstar ***
System going down IMMEDIATELY


Apr 27 04:07:40 shutdown: shutdown by root:

System shutdown time has arrived
panic: blockable sleep lock (sleep mutex) process lock _at_
/usr/src/sys/i386/i386/
trap.c:719
cpuid = 1; lapic.id = 01000000
Stack backtrace:
backtrace(c0331b7c,1000000,c0333c54,ce7a7994,1) at backtrace+0x17
panic(c0333c54,c03404d0,c0330093,c0345473,2cf) at panic+0x10a
witness_lock(c363c068,8,c0345473,2cf,0) at witness_lock+0xaf
_mtx_lock_flags(c363c068,0,c0345473,2cf,c2dda980) at _mtx_lock_flags+0x64
trap_pfault(ce7a7a8c,0,38,c01ed4f0,38) at trap_pfault+0x9d
trap(ce7a0018,c01d0010,c2460010,c2dda980,c0389da0) at trap+0x37d
calltrap() at calltrap+0x5
--- trap 0xc, eip = 0xc01dc667, esp = 0xce7a7acc, ebp = 0xce7a7ae0 ---
sched_choose(c038a430,c363b3c0,0,c363c0b8,ce7a7b54) at sched_choose+0x77
choosethread(c2dda980,0,ce7a7b2c,c01c1d82,d8b2364f) at choosethread+0x35
mi_switch(c2dda980,0,c0331480,1fc,ce7a7b88) at mi_switch+0x17d
_mtx_lock_sleep(c03875e0,0,c03400ef,d2,0) at _mtx_lock_sleep+0x1d6
_mtx_lock_flags(c03875e0,0,c03400ef,d2,c0391640) at _mtx_lock_flags+0x4a
vm_fault(c24ada00,bfbfe000,2,8,c2dda980) at vm_fault+0x5a
trap_pfault(ce7a7d48,1,bfbfee98,c363c000,bfbfee98) at trap_pfault+0xef
trap(2f,2f,2f,0,8104000) at trap+0x1f0
calltrap() at calltrap+0x5
--- trap 0xc, eip = 0x80527ec, esp = 0xbfbfee80, ebp = 0xbfbfeea8 ---
Debugger("panic")
<total freeze here, no debugger>