Great find! Thank you for sharing the OP on HN.
There's no evidence that "Agua Tofana" ever existed, and yet, for centuries, Europeans widely believed it was real, and widely feared it as a colorless, odorless, tasteless, undetectable, gradual-acting poison that could be added to anyone's food. Unscrupulous salespeople, as always, found clever ways to package and sell fake Agua Tofana -- similar to the fake snake-oil sold as "medicine" in the 18th and 19th centuries. The capacity of human beings to believe in things for which there is no evidence never ceases to amaze me.