Distilling a really toxic compound: ethyl fluoroacetate.
Fluoroacetic acid and related compounds are highly toxic. Any derivatives that contain fluoroacetate or degrades into fluoroacetic acid are highly toxic compounds with a LD50 as low as 0,25-2 mg/kg. This means that only a few drops of this chemical that can be seen flowing through the distillation apparatus is probably more than enough to cause serious problems.
The unusually high toxicity of fluoroacetic acid and of other monofluorinated organic compounds that can be metabolized to fluoroacetate has stimulated much research into the mechanism of this toxicity. This acid inhibits the enzyme, aconitate hydratase, which normally catalyzes the dehydration of citric acid. As a result, citric acid accumulates in the organism and the energy-producing cycle is interrupted. The formation of fluoroacetyl CoA and the formation of fluorocitric acid is often referred as lethal synthesis.