Reinheitsgebot
The German Beer Purity Law, which decrees that no other item should be used for beer other than barley, hops, water...and yeast, after the 17th century, when people realized yeast was a thing.
Reinheitsgebot turns 500 in 2016, making it the oldest, still valid food safety and consumer protection legislation.
Read this musing about Reinheitsgebot by Stan Hieronymus here and this Thrillist article about it here.