Long story, short: someone was assaulted with a hot cup of coffee yesterday, and I wound up discussing the impacts of caffeine on the skin.
Caffeine CAN be absorbed through the skin (which was a running joke in the Sylvia comic strip by Nicole Hollander, in which the protagonist started developing elaborate wardrobes for her cats after using caffeinated soap, and the cats begged her to stop), and is a fashionable item in cosmetics (it constricts blood vessels, and so can reduce some types of swelling).
But somehow, I wound up reading that caffeine is also a highly effective pesticide against invasive species of frogs in Hawaii. This makes some sense to me, based on my limited understanding of amphibians and their sensitive, not-especially-protective skin.