Since I write software that writes software for machine tools, I have extra opportunities to break things.
There's a technology called Electrical discharge machining, which means putting stuff close together in a fluid, running current through them, and having sparks burn off little pieces of material until you've got what you want. One manufacturer makes machines that have sophisticated programming, but it's not at all safe. Once, with the support guy from the company we got these from looking over my shoulder, I made a slight mistake that caused the arm of the EDM machine to slam against the metal we were machining, for a $16K repair.
Another time, a variable contained a Z level (height) that was used for two different things, but for everything we'd done up to then the two different things shared the same value. I was the guy who made the change that made the difference significant, and so some of our CNC mills thought the metal being machined was significantly lower than it was, so the setup moves for the machining that assumed the endmill was moving through air tried slamming through the metal. Some of the results were spectacular, although I never did find the cost.
Fortunately, at least for my self-esteem, people more experienced than me were supervising each of these mistakes, so I didn't feel too stupid, and my colleagues were very understanding.