The result of reverse-biasing an electrolytic capacitor. Consequences include but are not limited to an explosion of smoke, flame, and electrolytic goo complemented by a sharp "BANG."
Engineering Student A: "What smells like an electrical fire?"
Engineering Student B: "Dude, I think you put that capacitor in backwards."
Professor: "Did somebody just pop a cap in my lab?"
*Cue Students A and B snickering*