io9, my favorite site for all things sciencey and science ficitonal, has an article on the singularity - not the gravitational singularity that created space-time, but the technological one, the future event beyond which technology becomes incomprehensible to our current selves. The example they give is trying to explain the internet to a medieval peasant. Imagine a future technology so unthinkable that it can't be explained to you and me. Hmm, if it can't be thought of it can't be imagined, but of course that's what SF writers try to do.
The typical hypothetical event is the emergence of artificial intelligence - not because we can't imagine it, but because we can't really predict what effects it will have on society and, Star Trek TNG notwithstanding, on our very definition of what it means to be human. What would self-programing intelligence that surpasses human intelligence get up to in its spare time? Would it rather take over the world or play with Lego?