when trying to remove apache (mod_ruby gets remocved, too):

The following packages will be REMOVED:
  apache apache-common apache-manual mod_php mod_ruby mod_ssl
[...]
warning: /etc/httpd/conf/ssl/ssl.default-vhost.conf saved as
/etc/httpd/conf/ssl/ssl.default-vhost.conf.rpmsave
E: Sub-process /bin/rpm returned an error code (255)

the last line was generated by mod_ruby %preun:

trickster:~> sudo rpm -e mod_ruby
/usr/bin/subst: open: /etc/httpd/conf/httpd.conf: No such file or directory
error: execution of %preun scriptlet from mod_ruby-1.0.6-alt1 failed, exit status 1

...or at least check the existence, or suppress exit code and stderr output.