It backfeeds onto the grid and becomes the utility companies problem to deal with. This is similar to them dealing with a house that is an unbalanced consumer between the legs.

To my understanding, it is basically a non-issue at residential voltages. To be a problem it would require all the houses on a transformer (there are typically just a few for each transformer) to be unbalanced significantly in the same direction. That would be the end of it, too, because the 240V split only happens at the transformer.