In medieval times, the populous was terrified of dragons. Everyone knew someone who knew someone who had seen one. Many knew someone who knew someone who had lost a relative to dragons.
However, when they built the castle, usually in stages over decades, getting stronger with time, they always stopped long before the castle became dragon-resistant, much less dragon-proof. After all, dragons are awesome creatures; they are very strong and they fly. How high would the walls of the castle have to be to keep the dragon from just flying over?
So, they built their walls to resist bears and brigands. They fully intended to get around to resisting dragons but it was so expensive that somehow it never got done. After all, bears and brigands were much more numerous than dragons.
In modern terms, we would call the dragon strategy, risk acceptance. This is sort of like our strategy for greater than Richter 7.0 seismic events and greater than Saffir-Simpson Category V storms. Needless to say, the watch mounted the walls everyday, with their bows and arrows, ready to repel the dragons, but they never saw any.
Every twenty years or so we have a massive power blackout, embracing multiple states and tens of millions of homes, and lasting for several days. It is usually the result of multiple simultaneous failure of a highly unlikely number of components. The media and the politicians scream "There be dragons. Why weren't you prepared?" The industry says mea culpa and promises to do better next time.
Actually they do do better the next time. They raise the walls. They replace older components with new ones that have a longer mean-time-to-failure. They add redundancy so that they are better able to tolerate component failures, and they automate the response to component failures. Of course, all of this ads cost. Long before the mean-time-to-failure of the system reaches infinity, they stop.
In fact, in about twenty years, their best efforts will be overwhelmed once more. The knee of the curve that plots mean-time-to-massive-failure against cost seems to be at about twenty years. I have now lived through three such blackouts and hope to live to see a fourth. Mitigating it will be expensive but not as expensive as preventing it.
No matter how high we build the walls, the damned dragons just fly over.