Methanol; the poisonous byproduct of distilling drinking alcohol.
Crazy! Anyone who knows anything about Prohibition knows this is what killed lots of people in America back then. That and using lead distilling hardware like car radiators. It made some people blind too.
Depends on the source; fruits usually have higher concentrations of natural methanol. And since methanol has a lower boiling point than ethanol (148°F and 174° respectively), it can be separated in the distillation process by monitoring the still temperature.