I recall someone finding a Civil War shell and using it as a doorstop. Which was really cool until it blew up.
My favorite is some old revo war cannon shot that were submerged or somesuch in a certain kind of soil. The unstable exp. material had metamorphosed into something entirely different. They were on display for months/years in one of those triangle deals that cannon shot is always stored. And one day they started glowing red-hot. That just had to cause the proprietor to second-guess the decor!!!