DHCP client can fail permanently if an interface is down at boot
If when the WAN is brought up at boot launching dhclient the interface is down it will fail and stop:
pfSense php-cgi: rc.bootup: The command '/sbin/dhclient -c /var/etc/dhclient_wan.conf igb0 > /tmp/igb0_output 2> /tmp/igb0_error_output' returned exit code '1', the output was ''
The output there being:
igb0: no link .............. giving up
If the WAN then comes up before bootup has completed it does not trigger dhclient to re-run leaving the WAN permanently down after boot.
This can happen typically if the WAN is connected to a modem and there is a power outage causing both devices to boot at the same time.