Removal of vixie-cron in Red Hat Linux 7.2 (and probably earlier) fails
if cron is not first running. The preun script looks like this:

if [ $1 = 0 ]; then
    /sbin/chkconfig --del crond
    /sbin/service crond stop >/dev/null 2>&1
fi

It should be more like the following:

if [ "$1" = 0 ]
then
	/sbin/service crond stop > /dev/null 2>&1 || :
	/sbin/chkconfig --del crond
fi