Bug#605535: apache2.2-common: a2dissite bash completion cannot cope with 000-default/default site
It seems that you've added code to a2dissite/a2ensite to nicely handle
the special(?) sites "default" to be added automatically as "000-default".
Both tools also provide bash-completion, but a2dissite only identifies
the "000-default" site, not the "default" original name.
Perhaps (but not sure whether it's really worth it) this might be added.