Calabar Beans: Pre-History's Lie Detectors
The Efik people of the region that is now Nigeria used to force people accused of crimes to suffer a trial by ordeal: they'd be fed calabar beans, a known poison. If the accused died, they were judged guilty. If they lived, they were "proven" innocent. There's some pharmaceutical basis to this. It turns out that the poison of the calabar bean is absorbed in the mouth, where a guilty person might try to hold the beans, to avoid swallowing. For the guileless who swallowed them whole, the emetic properties of the beans might cause them to throw up the beans and escape poisoning.
Calabar beans aren't just poisonous; a compound from them, physostigmine, has been used to treat glaucoma and Alzheimer's disease.