Play a single player map of type "Jungle". There has to be a neutral creature "Bangalaas" on the map. Go to it with some unit, select the creature and start repeatedly clicking on it with the left mouse button (Check your APM, at least 5 clicks a second). After several seconds (maybe 10, or 20 secs) the creature explodes with atomic effect. Weird.
It's intentional, it's a fun joke of critters exploding when you click them repeatedly that started in Warcraft 1