After hearing about so many historical experiments that disregard patients' rights, it does me good to finally find that one - regarding the effects of coffee - proved fatal for everyone but the patients.
Coffee has taken over every country it ever has reached, but in some countries it encountered a lot of resistance. Arriving in Sweden in 1674, it hit the big time became the get-together drink of Swedish intellectuals. Sitting around a pot of coffee they'd discuss the great philosophical questions of the day, like which was the best meatball recipe, and how to wear candles on your head without setting your hair on fire. (I may not know a lot about Swedish culture.)
Coffee was popular, but not with everybody. King Gustav III ascended to the throne a good three-quarters of a century after coffee came to Sweden. He was eager to turn back the tide. He believed that coffee was a poison, and that people needed to be informed about its terrible effects. To that end, he proposed an experiment.
Gustav located twin brothers who were facing a death sentence, and offered to have their execution commuted to life in prison. There was one condition. One brother would have to drink three pots of coffee a day. The other would drink three pots of tea. Gustav, aided by two physicians, would monitor the twins' health. Surely, Gustav thought, the coffee-drinker would be the first to succumb to this lethal poison.
Unfortunately for Gustav, he found that the one thing more lethal than coffee was being named king of Sweden. He was shot during a masquerade, and died well before either the coffee or tea drinker. Things began to get creepy when both the doctors monitoring the experiment also died, leaving the condemned twins to keep being stuffed full of caffeinated beverages without proper supervision. In case you're wondering, the tea drinker died first - at the age of 83. No one knows the date of the coffee-drinker's death. For all we know, he's still wandering around today.