The Royal Institution posted this demonstration of an explosively unstable substance called nitrogen triiodide. I love the purple smoke it makes.
Nitrogen triiodide is so unstable that even something like a mosquito landing on it can set it off. Three iodine atoms cluster around one side of a nitrogen atom. Being crowded around one end causes something called bond strain as the atoms repel each other in a small space. The result is that the molecule is prone to falling apart, explosively.