hydrofluoric acid. Get some on your skin, and you don't even feel a burn. But within minutes or hours, you may have demineralized your bones, causing them to go beyond brittle

Dimethylmercury, for example. https://en.wikipedia.org/wiki/Dimethylmercury

True...but Hg(CH3)2 is so much of a "MSDS from the 11th circle of hell" corner case that only expendables in heavily armored moon suits should ever go near the stuff.

Yet a German synthesized it in his basement and used it to poison a colleague: https://www.theguardian.com/world/2019/mar/07/german-who-poi...

A crime as heinous as the substance that was used. It's a miracle that he didn't kill himself or one of his family members with it.