Found in Andrea Wulf's review of Steve Jones's book No Need for Geniuses. There was once an opponent of the death penalty who proposed a painless method of execution. His name was Joseph-Ignace Guillotin. For me, that pretty much defines the unintended consequence.
Wulf's review, entitled Let them eat potatoes, appears in the Life & Arts section of the Weekend FT, 2/3 April 2016, The full title of Jones' book is No Need for Geniuses: Revolutionary Science in the Age of the Guillotine. And a neat detail of the story (quoting from the review) is that Joseph-Ignace's family "changed their surname in protest when thousands were executed by guillotine".
Wulf's review, entitled Let them eat potatoes, appears in the Life & Arts section of the Weekend FT, 2/3 April 2016, The full title of Jones' book is No Need for Geniuses: Revolutionary Science in the Age of the Guillotine. And a neat detail of the story (quoting from the review) is that Joseph-Ignace's family "changed their surname in protest when thousands were executed by guillotine".