Single user mode exits unexpectedly

[Janos Dohanics]

I have just rebuilt world and kernel according to the Handbook,
installed the new kernel, rebooted, logged in, issued "sudo shutdown
now" - the machine entered single user mode, then immediately exited
without any intervention by me and continued to boot into multiuser
mode.

Here is a snippet from /var/log/messages:

Dec 30 17:41:15 iguana rc.shutdown: 30 second watchdog timeout expired. Shutdown terminated.
Dec 30 17:41:15 iguana init: /bin/sh on /etc/rc.shutdown terminated abnormally, going to single user mode
Dec 30 17:41:15 iguana syslogd: exiting on signal 15                    <---
Dec 30 17:41:28 iguana syslogd: kernel boot file is /boot/kernel/kernel <---

This seems to be happening every time in response to "shutdown now".

However, I can cold boot this machine into single user mode with
nothing unusual.

This is FreeBSD 8.2-STABLE amd64, rebuilt on 12/26/2011

I guess I may have unintentionally changed a config file? Where should
I look?

-- 
Janos Dohanics