Water usage is probably my biggest. Living in a high desert, my wife and MIL see no problem with filling one side of the sink with hot soapy water to wash a few dishes because “that’s just how I’ve always done it”, to watering the grass and plants for hours. All of this makes me mental.
To secure your stuff. Hacking and social engineering are both easier than you think and identity theft is devastating. I know most people here already know that, but having worked in tech support, it’s unbelievably common to skip even the basics, not to mention my grandparents who think I’m just paranoid.