The danger in leaving a machine on, unattended, is that if something gets put onto the foot pedal, it can burn out the pedal or the motor, or cause a fire -- all because something got put onto the pedal. I've seen foot pedals melted to the carpet, and motors burned to a crisp, and the owner was glad that there wasn't a fire in the house. Even if the pedal is pushed slightly, and not even moving the machine, there's still an electrical current going through the pedal, and into the motor. So be careful -- especially on the older machines!