I nuked myself.
One of the first games I won I just had to build every unit and see what it it looks like and what its range is and such, whether I needed them or not. I built my first ICBM, hit the bombard button thingy, and wow, the whole world is in range! Cool!
Only then I couldn't figure out how to turn it off. There was nowhere I could click to disable the targeting, unlike with bombers or artillery, for example, where you can just click outside their range. I couldn't figure it out. Finally I clicked on the city containing the ICBM, figuring that square at least must be disabled.
Nope. Big mushroom cloud, lots of orange splotches, half-population city.
Oops.
Renata