Well, there's a phoenomenon called "flash over", that realizes in a confined environment, and happens when the temperature of a fire becomes so high that all the substances within starts to burn and feed the reaction.
Now, imagine that the whole world could become a closed environment for the flashover...
So we should stop using fire until we prove that the world will not burst into flames?
http://fusion.net/story/54583/the-case-against-killer-robots-from-a-guy-actually-building-ai/
(An earlier version of this article was titled 'Andrew Ng disses UFAI concerns" which is the phrasing many of the commenters are responding to).