Comment 1 for bug 240098

the error comes from /etc/init.d/slim script, after attempting to issue 'stop' (or restart)
slim not responding to TERM singals...

$ tail -1 /var/log/messages
slim: unexpected signal 15

as reported, after the failed stop, slim does not respond to user input anymore

only way to kill it , is the hard way:
killall -9 slim

bad news for ubuntu/debian users :(