RELENG_5: occasional panic on shutdown

Once in a while I drop into ddb when I try to shutdown my RELENG_5
machine from a serial console; please see below.

Is there anything interesting I could do in ddb to diagnose the problem?

$ sudo shutdown -h now
Shutdown NOW!
shutdown: [pid 522]
$

*** FINAL System shutdown message from thomas_at_current.tmseck.homedns.org ***
System going down IMMEDIATELY


Sep 11 20:50:15 current shutdown: halt by thomas:

System shutdown time has arrived


Fatal trap 12: page fault while in kernel mode
fault virtual address   = 0x1c
fault code              = supervisor write, page not present
instruction pointer     = 0x8:0xc04c16cf
stack pointer           = 0x10:0xcc551784
frame pointer           = 0x10:0xcc551790
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 251 (syslogd)
[thread 100038]
Stopped at      knote+0x27:     cmpxchgl        %ecx,0x1c(%edx)
db>