CNC milling with open source software
CNC is programming "for keeps".<p>Any tiny little error in your gcode has a chance to crash the machine.  Unlike a computer crash, a CNC crash results in broken metal flying across the room and anywhere from $5- to $5000 in damage.  I programmed for a heavy machine shop for a year before I gave up and went back to making websites.<p>Hobby CNC is fun though, it is amazing what you can make out of wood when you can shape it with sub-millimeter precision. Also hobby machines are not strong enough to cause serious damage when you crash.