pfSense

After going through the setup wizard with a single interface assigned, you're left with a broken configuration. LAN is added to the config as this:

                <lan>
                        <ipaddr/>
                        <subnet>1</subnet>
                </lan>

Which drops LAN into the firewall ruleset even though it isn't assigned to an interface, breaking the ruleset amongst other things.

Likely best resolution: if only one interface is assigned, skip the LAN configuration page.