"Flash in the pan"

In the 17th century, the pan of a musket was where you put the powder that was ignited by the sparks from the flint. If the powder ignited properly, the flash would set off the charge in the gun, firing the ball out of the barrel. Occasionally, the priming powder would burn without igniting the main charge. The burn was visible but ineffectual, just as a flash in the pan is successful, but only briefly.