You could imagine scenarios of hackers taking control of ovens, fridges, etc. and turning them to consume just a little more electricity en-mass with a combined effect that could catch the national grid off-guard and crash the system.
As we've seen, the people making the gadgets that make up the "internet of things" have network security very low on their lists of priorities (WiFi and cable routers are a shockingly insecure example). It just doesn't immediately affect their bottom line and so they're reluctant to spend time, effort, and resources on it.
I reckon it'll take international conventions and effective legal regulation to get botnets under control; Something that Silicon Valley and their venture capitalist backers are allergic to. They don't want no big gubbermint.