Since the basic software will not be touched, I figure you would end up with a non-bootable ROM, but you should still be able to get into Service Mode and reflash the ROM.
As said, it will refuse to flash without a AC adapter, but I can imagine a pet, kid or wife stepping on the cable and ripping the plug out, while somehow your battery is dead as well, just as you're flashing a fresh ROM
Apart from that, there's always the full NAND restore function, which is, as far as I know, completely hardcoded in some chip, which enables you to flash a NAND image just like on the 5500, by holding a few keys.