New Scientist has two articles today concerning what I see as potential causes for TEOTWAWKI (The End Of The World As We Know It).
Rise of the garage genome hackers is about people making new biological organisms in their garages. I think it's fairly obvious how that could go wrong, even if it is unlikely.
Comet smashes triggered ancient famine tells us about a series of comets that hit the earth in 536 AD and triggered 18 months of darkness that caused world wide famine. Even in 536 AD, when we weren't so detached from our environment and dependent on technology for continued survival, that must have been very much like the end of the world.