Grey goo scenario
From Flexible Survival
Basic Information
The Grey Goo Scenario is one of many terrifying possible doomsday events that have been 'predicted' by science fiction authors over the years. It concerns the mistaken unleashing of badly-programmed or damaged self-replicating nanites which then run amok, turning everything into more of themselves. Thus, eventually everything on Earth will be a uniform grey goo.
Unfortunately... It stopped being science fiction when a version happened in Las Vegas, Nevada when the Promethean Nanites were released. Vegas is dead, and few survived to tell the tale.