userdel dumps core (memory fault) if user's group doesn't
exist. Yes, this is bad usage but may confuse the heck out
of someone.

howitzer:/root>useradd foobar
howitzer:/root>groupdel foobar
groupdel: cannot remove user's primary group.
howitzer:/root>vi /etc/group  # removed group dingus
howitzer:/root>userdel foobar
Memory fault (core dumped)
---Eof---