A smart thermostat can cycle a furnace on & off at high frequency and cause it to break down. Same with smart light controls. If there are specific models tha

A smart thermostat can cycle a furnace on & off at high frequency and cause it to break down. Same with smart light controls. If there are specific models that burst into flames when you do this, then you can cause half a town to catch on fire at once. You could also hack all the smart thermostats at once and cause annoying brownouts due to crazy simultaneous demand happening all at once. You can also cause peaker plants to go on and off at an annoying unpredictable frequencies by making these spikes short. But to be honest, I doubt anyone will do this, since most vital infrastructure is relatively easy to sabotage and nobody has done it. A group of 100 organized assholes could do horrible damage to the US power grid today by going out to the middle of nowhere and blowing up a bunch of high voltage power transmission towers.