A brand-new power strip emitted a loud pop and left a soot mark on my friend's hand when he plugged it into the wall. No devices were attached to the power strip. I decided to take the thing apart to see what failed. The circuit breaker in the household electrical system did NOT trip during this event.
My guess is that a very fine piece of wire bridged the neutral and hot terminals on the switch. When power was applied, the whisker vaporized.