I think the thing I'd be worried about first is the transformers. Would they handle that much power flowing in reverse without overheating/catching on fire? The

I think the thing I'd be worried about first is the transformers. Would they handle that much power flowing in reverse without overheating/catching on fire? They're already one of the weaker parts of our system; they don't handle overcurrents well (they're largely what folks are concerned would fail due to a solar flare). The second thing would be all the sensors attached to the grid. Would they identify a sudden and dramatic change in flow as a fault, and throw their switches? Also, where would that supply of energy flow? If there's not enough of a sink for that electricity, what happens to the generators? If the sink is outside the local area, what would happen to those transmission lines?