Orphaned rules from deleted interfaces are still present in config
When you delete a normal interface, the rules are left orphaned in config.xml without an interface tag so there is no easy (Read: In the GUI) way to delete or reassign the rules. Among other problems, if one of these rules used an alias, you can't rename or delete the alias since the system believes the alias is in use.
When a normal interface is deleted, these rules should be deleted as well.
Note that this is not that case for PPTP, IPsec, and OpenVPN. Those rules keep their interface association when their VPN type is disabled, so they can safely be left.
#2 Updated by Jim Pingle over 8 years ago
You are right the interface deletion code does get rid of the rules, but if someone deleted the interface before that code was added, or they upgraded from 1.2.x with the orphaned rules, they could still cause problems. We may need a bit of upgrade code to reap the old orphaned rules without interfaces set.