Most of the time I ran:

service nginx restart

I would get this message in the /var/log/nginx/error.log file:

panic: MUTEX_LOCK (22) [op.c:352].

After some hunting around, it appears to be a known bug in nginx (perhaps perl in nginx?)… Anyway, a simple workaround is to do this:

service nginx stop
service nginx start

Or, simply edit /etc/init.d/nginx, and add the sleep 1 line:

51 restart() {
52     configtest || return $?
53     stop
54     sleep 1
55     start
56 }

Nice workround!