Came across this concept in a blog comment1, and went down a google rabbit hole. This piece was one of the better intros to the concept, which comes from Nassim Taleb in his book Antifragile. I'll let Taleb explain it:
To take one example, risk management professionals look in the past for information on the so-called worst-case scenario and use it to estimate future risks – this method is called “stress testing.” They take the worst historical recession, the worst war, the worst historical move in interest rates, or the worst point in unemployment as an exact estimate for the worst future outcome. But they never notice the following inconsistency: this so-called worst-case event, when it happened, exceeded the worst [known] case at the time.2
It's a logical fallacy, but an understandable one -- we base our thoughts on politics and economics on what we've seen and experienced, either individually or collectively. And Taleb isn't encouraging blind pessimism, just an awareness that risk in any situation may be greater than we've realistically accounted for.
Of course, part of the reason Bitcoin exists is as an answer to exactly this sort of thing -- we tend to consider economic outcomes that go beyond the "normal" risk levels much of the world envisions. But in any situation, it's a good concept to keep in mind and look to when analyzing risk.