One of the more popular issues today of the classic US commemorative coin series is the 1937 Battle of Antietam 75th Anniversary Half Dollar. Its US Civil War theme is a big driver of this popularity, and its strong aesthetics certainly help. The coin was not, however, hugely popular back in 1937.
The enabling Act for the Antietam coin authorized the striking of up to 50,000 silver 50-cent pieces "in commemoration of the seventy-fifth anniversary of the Battle of Antietam." The coin was sponsored by the Washington County Historical Society of Hagerstown, Maryland; the Society had high hopes for its coin.
Contemporary advertisements for the coin boasted:
"Because of the world-wide fame of General Robert E. Lee, Commander-in-Chief of the Confederate Armies, and the outstanding place General George B. McClellan holds in history, the commemorative coin is certain to become a rare treasure."
The half dollars were struck by the Mint in Philadelphia in August 1937 and were promoted via news releases and advertisements in hobby magazines. The coins were priced at $1.65 each (including postage and insurance), which was in line with other issues of the time.
Unfortunately for the Society, the coin struggled to find buyers as its launch came on the "wrong side" of the commemorative coin boom/bust of 1936. With collectors staying away from the issue (and commemorative coins in general), the Society was forced to return 32,000 of its coins to the Mint to be melted. The coin's 64% melt rate left a net mintage/distribution of 18,000, a level that appears to meet today's collector demand as the Antietam's market price, while several hundred dollars for nice examples, is not outrageous.
1937 Battle of Antietam 75th Anniversary Half Dollar

For other of my posts about commemorative coins and medals, including other Melting Pot stories, see:
Commems Collection.