At boot time there is sometimes to little time for dhcp negotiation even if DHCLIENT_WAIT_AT_BOOT and WAIT_FOR_INTERFACES are sufficient big. This problem occurs if an interfaces is created while rcnetwork is waiting for mandatory devices. 
In this case ifup is running in background (called from udev) so DHCLIENT_WAIT_AT_BOOT has no effect. rcnetwork does only check the state of this interface via ifstatus.
And the counter for WAIT_FOR_INTERFACES is started when rcnetwork is started. Now when the interface is created n seconds before rcnetwork times out, then there are just n seconds for dhcp negotiation before rcnetwork reports failed and booting proceeds. (E.g. with starting ypbind.)

Severity is minor, because dhcp negotiation proceeds in background. Often the interface will be set up in time despite rcnetwork reported failed.