Also known as the grey slime scenario or the Grey Goo End. A worst-case state in nanotechnology development. The scenario:self-replicating nanotech 'machines' are created and have the ability to restructure matter at the atomic level. Due to an uncontrolled release or design flaw, the machines restructure all available matter into more nanotech machines, covering the earth in a grey slime of nanobots. Similar in concept to Kurt Vonnegut's chillin' Ice-9 disaster at the end of Cat's Cradle but with a hip nanomech twist.

Just so you don't spend a sleepless night worrying, rest assured that available energy constraints and ready supplies of necessary elements would probably keep this from happening. However, some more likely goo scenarios are

On the upside, there might be a defence in the form of blue goo.