Refuted in the 18th century by Lavoisier. Blondlot could see in a darkened room. Debunked during an exhibit in 1904 when a visiting scientist surrepetitiously removed a prism from the spectroscope from which Blondlot was taking measurements, said removal not having any effect on said measurements.
Enthusiasts claim there are two million Esperanto speakers worldwide.
Still a failure. Susan B. Anthony dollar coins were minted from 1979-1981, and looked too much like a quarter for anybody to be comfortable with them. The failure has to have been somewhat notorious, so that the average reader will have heard of it. For the purposes of this page, the average reader is me. If it's a product, it has to be no longer obtainable.