Flash in the pan
Something which disappoints by failing to deliver anything of value, despite a showy beginning.
There's reason to believe that this phrase derives from the Californian Gold Rush of the mid 19th century. Prospectors who panned for gold supposedly became excited when they saw something glint in the pan, only to have their hopes dashed when it proved not to be gold but a mere 'flash in the pan'. This is an attractive and plausible notion, in part because it ties in with another phrase related to disappointment - 'it didn't pan out'. 'Panning out' can be traced to US prospectors and was used in that context by the early 20th century; for example, Paul Haworth's Trailmakers of the Northwest, 1921:
"The Colonel had told them that a cubic foot of gravel would pan out twenty dollars in gold."
Nevertheless, gold prospecting isn't the origin of 'a flash in the pan'. The phrase did have a literal meaning, i.e. it derives from a real flash in a real pan, but not a prospector's pan. Flintlock muskets used to have small pans to hold charges of gunpowder. An attempt to fire the musket in which the gunpowder flared up without a bullet being fired was a 'flash in the pan'.
The term has been known since the late 17th century. Elkanah Settle, in Reflections on several of Mr. Dryden's plays 1687, had this to say:
"If Cannons were so well bred in his Metaphor as only to flash in the Pan, I dare lay an even wager that Mr. Dryden durst venture to Sea."