What have been your worst cases of hardware failure?

A local business had a "cow" computer and purchased a new monitor for it. They called to have me install it, and fix a couple other things in their office.

When I plugged in the monitor and turned it on (even before I turned the computer on) it started smoking. The monitor filled the small room with smoke before it fizzled out.

That incident drew quite a crowd. I didn't know that there were that many people in the building.