As far as I know, if the iSight is plugged in, it's powered on, whether or not the shutter is closed, so there's no such thing as "powered off".
If we're moving here, rather than to support, what I said to this message originally was:
It's hard to say for sure, but it certainly seems that something went awry with your FireWire bus when this occurred. iSights, for example, are well known for causing bus resets and voltage fluctuations, which can cause errors during backups. Usually, isolating them on their own hub helps, but there's no guarantee.
In general, when you see this kind of thing, I'd shut everything OFF (drives/hub) and restart. That will often fix things up... but there's little else that can be done until every device and software subsystem is bug free!
