error during shutdown process

My 7.0-CURRENT box got a strange error message during shutdown process
since some time. Don't know exactly when it started. See the following
for details.

# uname -a
FreeBSD  7.0-CURRENT FreeBSD 7.0-CURRENT #4: Wed Aug 17 15:51:06 CEST
2005     acex5_at_:/usr/obj/usr/src/sys/GENERIC  sparc64
# init 0
Shutting down daemon processes:.
Stopping cron.
Shutting down local daemons:.
Writing entropy file:.
Terminated
.
Alock order reversal
 1st 0xc045f4b0 Giant (Giant) _at_ /usr/src/sys/kern/uipc_usrreq.c:813
 2nd 0xc04ee568 udp (udp) _at_ /usr/src/sys/netinet6/udp6_usrreq.c:674
KDB: stack backtrace:
_mtx_lock_flags() at _mtx_lock_flags+0x6c
udp6_detach() at udp6_detach+0x20
soclose() at soclose+0x260
soo_close() at soo_close+0x4c
fdrop_locked() at fdrop_locked+0x9c
fdrop() at fdrop+0x20
closef() at closef+0x3c8
fdfree() at fdfree+0x534
exit1() at exit1+0x490
sys_exit() at sys_exit+0x10
syscall() at syscall+0x2dc
-- syscall (1, FreeBSD ELF64, sys_exit) %o7=0x4052f384 --
userland() at 0x40530ee8
user trace: trap %o7=0x4052f384
pc 0x40530ee8, sp 0x7fdffffce21
pc 0x104524, sp 0x7fdffffcee1
pc 0x1063c0, sp 0x7fdffffd011
pc 0x101d94, sp 0x7fdffffe4f1
pc 0x40211474, sp 0x7fdffffe5b1
done
Waiting (max 60 seconds) for system process `vnlru' to stop...done
Waiting (max 60 seconds) for system process `bufdaemon' to stop...done
Waiting (max 60 seconds) for system process `syncer' to stop...
Syncing disks, vnodes remaining...1 1 0 0 done
All buffers synced.
unmount of /dev failed (BUSY)
Uptime: 4m53s

System is powered of after the last message. Can't see further
disturbances. Maybe something went wrong in code cleanup of file
src/sys/netinet6/udp6_usrreq.c three weeks ago.

Best Regards,
Sebastian