Two Cambridge undergraduates inventing the Sandbox Open Ended and 3D Space Simulation genres by themselves in their spare time. Funnily enough, they were initially rejected because their design was too radical and away from the "3 Lives and 10 minutes of gameplay" trend of the time.
Genius Programming: The amount of innovations and creative programming needed to implement a groundbreaking game of this magnitude in only 22ks, a tiny fraction of what's avalaible to the simplest phone. Braven gives a not too technical lecture in this 2011 video.
The original game had so many features and feelies that it used every avalaible bit, so in the end it figuratively AND literally overflowed its box.