Its already happened hundreds of times during research, testing and at least twice during war. Nefarious countries with powerful weapons? What could go wrong???
The ugliest weapon, in my opinion, was made in the cold war. Its declassified, there are documentaries on it. "Project pluto". Imagine a nuclear powered drone rocket - powered by an open air reactor, spewing radiation out the back for months on end, killing indiscriminately, flying at mach 3 with sonic booms causing damage the entire time. Also carrying multiple nukes, dropped at targets, and when the reactor stops producing heat, the missile itself crashes and itself is another large nuke.
Terrible things we have created. Just terrible. We really did not like those ruskies.