The SCI 301 Puzzler

After a hurricane, a homeowner connects a portable gasoline generator to the service panel as shown. The green backplane is the neutral (white wire) and the red is one of the two phases in the house. The homeowner first carefully turns off all appliances inside the house, so that there will be no initial load on the generator. But when she starts the generator, considerable current is immediately drawn from the generator.

What are the possible causes for this problem? There are a few obvious possibilities, but don't rule out anything plausible.

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