Removing USB keyboard after filesystems synced causes panic with destroyed mutex twa(4)?

Hi Alexander and Hans,
    I recently did the following which generated a panic on a
9-CURRENT kernel compiled on the 26th:

1. Executed reboot
2. Removed keyboard.
3. Some time after `All buffers synced\nUptime: ...' was displayed,
the keyboard was registered disconnected.
4. The interrupt was delivered to my twa(4) enabled card and the
kernel panicked, like so:

ugen2.2: <Mitsumi Electric> at usbus2 (disconnected)
uhub8: at uhub2, port 1, addr 2 (disconnected)
ugen2.3: <Mitsumi Electric> at usbus2 (disconnected)
ukbd0: at uhub8, port 3, addr 3 (disconnected)
uhid0: at uhub8, port 3, addr 3 (disconnected)
panic: mtx_lock_spin() of destroyed mutex _at_ /usr/src/sys/dev/twa/tw_cl_intr.c:88

cpuid = 1
KDB: enter: panic
[thread pid 12 tid 100025 ]
Stopped at         kdb_enter+0x3d: movq     $0,0x40289c(%rip)
db>

    I wish I could provide you with more details, but unfortunately I
the USB bus isn't registering the fact that I'm reattaching the
keyboard right now and the box won't reboot automatically :( (didn't
set the right sysctl beforehand to panic automatically). I'll try and
reproduce the issue again, but I was just wondering whether or not you
guys had seen this problem before.
Thanks,
-Garrett