One of the most important lessons I've learned during the growing-up phase of my being an entrepreneur -- and one of the first things I'm doing with my next startup -- is putting together a list of "doomsday scenarios" right off the bat.
What are the _single_ actions / events / scenarios that could kill your business overnight?
I emphasize single because every business is going to have the potential for a series of unforeseen events kill them (i.e. Microsoft would probably not have seen 'Apple releases a phone' as a business/Balmer-killer), but every business should be keenly aware of the single-points-of-failure (SPoF).
It seems obvious, but so many folks either willfully ignore or choose to pretend that SPoF don't exist or rationalize the scenario away.
Examples:
* You run on AWS in a single availability zone, and that availability zone goes down for a week ("The cloud is super reliable. Amazon won't go down.")
* Your CTO is the only person on the company who knows the product, and he quits overnight / gets injured / becomes incapacitated. ("He's my partner. He'd never abandon the business. We're in this together.")
* You rely solely on Facebook / Twitter / Apple App Store and are at the mercy of their changes in policy ("Oh Facebook would never just shut us down.")
* You're built solely on the Foursquare Places API. ("Foursquare won't just kill all the companies that rely on their API...right?")
Etc. etc. etc.
Now, this isn't saying that you should be paralyzed by the potential for SPoFs to hurt your business -- it may make sense to proceed even with them around (i.e. lots of people have made a lot of money building FB apps).
But you MUST be aware of what they are, and you MUST have a plan in place to handle the continuation of the business if one of them comes true.