Nassim Nicholas Taleb wrote a fantastic book on the impact of large, high impact, statistically rare events called The Black Swan: The Impact of the Highly Improbable. According to Taleb, history tends to swing much more than we realize on these theoretically foreseeable, but often unexpected large events as opposed to the much more predictable trajectory that we tend to forecast when we look ahead to the future.
Technically, Taleb would probably classify the events we're about to discuss as "grey swans," unlikely events that we should still be able to anticipate, but that's splitting hairs. Long story short, we should be aware that our world can change for the worse in a hurry and we should make sure we're taking steps to alleviate the danger.
Default on the Debt: This is, far and away, the scariest option on the list because all we have to do to ensure that it occurs is maintain the status quo for the next 10-15 years -- if that long. Congressman Mike Pence says we may only be a decade away from a default and Senator Tom Coburn thinks we may be less than 5 years away. By 2020, it'll take 19% of the world's GDP just to finance our debt and by 2030 it'll take 36% of all our tax revenue just to pay the interest on the debt. When people call our spending "unsustainable," that's not hyperbole. What happens if America goes bankrupt and defaults? A Depression would begin, your life savings could become worthless overnight, your taxes would explode, your standard of living would plunge, violence and instability would break out, and payments from the government could dramatically drop or stop altogether.