When networks depend on other networks, such as a communications network that relies on a power grid, failure can cascade back and forth between the two. This behavior may explain sudden breakdowns in interacting systems. Thus, the effects of an attack on a single node can reduce an übernetwork that starts with 12 operating nodes to just four.
Once studied solo, systems display surprising behavior when they interact.
Via FuturICT, Complexity Digest, Chen Quan