I'm one of those people who sits in bed at night planning for unlikely events. I know for instance my survival strategy in case I'm one of the lucky survivors after 99% of the worlds population is wiped out by a deadly Virus. I also have plans I've made in my head in case of Zombie invasion (well ok, I've decided where my strongholds might be and who I'm going to push in the way of a Zombie attack and who I'd want to save). I'm undecided about what I would do in the event of alien invasion; I guess it really depends on which model you subscribe to.
Of course these are all unlikely senarios. Far more likely is the taking of over of humanity by sentient machines ala Terminator Judgement Day. My only adjustment to this scenario is that I believe Skynet already exists and is called Google. Think about it, Google knows everything we know, it has mapped our planet (and military), it has started building it's own power source and laying down fiber networks across America (AKA a nervous system!) - it's even started tagging us so it can follow us everywhere with it's Android phones.
Anyway, I digress:
One scenario I hadn't considered was asteroid impact; mostly because I always thought if an asteroid hit we'd be buggered. Turns out it depends on many variables which you can calucculate at this lovely impact: earth website. Hoorah! Tonight I shall sleep well while I plan for various deep impact scenarios.