Flash in the pan

Means temporary or ephemeral success and has been used in this figurative sense since the late 18th/early 19th century. Its earlier literal sense derives from the 16th century when a flash or flare-up of the igniting powder in the firing pan of early firearms could sometimes occur but without a successful discharge of the weapon. It has nothing to do with panning for gold or primitive flash photography as is sometimes maintained. See also Half-cocked.