I used Webmin/networking to modify the port number on an INPUT rule to a new value.
First, hit "Revert configuration" to make sure I had an copy of the active firewall. Modified the rule with the new port value and saved. Hit "Apply configuration" and it gave an error (did not write it down. I think it was from an iptables command.) Went back to the prior page to see what was going on.
At this point, ALL the firewall rules were gone. Hit "Revert configuration" again, and no rules came back. Went into a bash command line, to find that /etc/sysconfig/iptables was also reset to nothing.
This is not acceptable for such a critical feature. If you get an error applying an iptables configuration, it should just revert to the old configuration, not wipe out the active and boot firewalls. It took me days to rebuilt the firewall from memory.
I would also recommend that prior to an "apply configuration," you should save a backup somewhere, perhaps /etc/sysconfig/iptables.backup just in case.