I had a Southwest Technical Products power amp that I built from a kit. There was some kind of intermittent short circuit which would put a lot of current through a ~1W resistor, causing it to glow orange and stream smoke up through the perforations in the metal case until I turned it off... then the problem would go away for a few weeks. In hindsight I guess it could have been ultrasonic oscillation or something, but at the time it just looked cool. The smoke resulted in a couple of "if you're taking drugs we hope you would tell us" lectures from my parents, however.
The amplifier continued to work with no audible change even when the smoke was pouring out.
EDIT - found a pic of the amp. The smoke came out of the lower right rear corner IIRC. Before you ask, the OVERHEAT light did not come on.http://www.swtpc.com/mholley/Catalog6/Page05.jpg