flash in the pan a thing or person whose sudden but brief success is not repeated or repeatable.

☞ This phrase developed from the operation of a flintlock or matchlock firearm, when the flash from the priming gunpowder in the 'pan' failed to ignite the charge in the barrel, so that the gun did not fire.

2014New Statesman Some have been celebrities for a long time, drawing the devoted to kiss their knees (more of that later). Others are mere flashes in the pan, so to speak.