I try to keep a general list of "black swan"-type events in my head, because aggregating exceedingly unlikely but highly impactful events generally means that one of them will actually happen. My criteria for events are those that are "game-changing". Things like the mortgage crisis and Hurricane Katrina were significant, but in the long run, (presumably) recover. I'm talking sudden, generally unexpected BIG things that reshape the face of an industry/field, the country or the face of the world for decades or even centuries.
On a world scale, the extinction of the dinosaurs was a black swan. Alexander the Great and Julius Caesar were black swans. Jesus and Mohammed were black swans. The wheel, written language, the printing press, electricity, the light bulb, the combustion engine, refrigeration, the semiconductor and the internet were black swans. X-rays, penicillin and sanitation were black swans. Horace Mann's public school was a black swan (for the United States). The reinvention of republican democracy in the US was a black swan. World War 1 was a black swan. These type of events often shape history, and given that we're currently on top, that is likely only bad news for us.
Black swans in individual fields happen, also - the world of ornithology (I'm told) was rocked by the discovery of black swans in Australia, which is where the term got its name. Whether or not you think cellphones really did completely change the face of the world, they certainly changed the face of telecommunications. Et cetera.
These kind of events are often not prepared for to the level they should be, because people perceive their likelihood as zero while decision-making, even if they acknowledge it separately. I group them in terms of "what preparation the world should be doing", which is why all of the events are negative.
My current list reads:
Warfare of Mass Destruction
Derivative-related economic meltdown from lack of diversity in bets
US government debt default
Yosemite mega-volcano eruption
Global warming-related melting of Siberian permafrost
Depressing, I know, but policies and budgets need to be aware of these so that preparations can be made.
In any case, there are always black swans that DON'T happen because of excellent prevention. Nuclear war was narrowly averted (arguably) during the Cuban Missile Crisis. The Y2K bug was anticipated and avoided. Again, et cetera.
Which is why it's always nice to see the world make a little progress at avoiding disaster.
The way nuclear bomb manufacture relates to nuclear power plants is a little interesting. Nuclear power plant fuel (for all but a few specialty Thorium and Plutonium nuclear plants) is a combination of isotopes of Uranium - U-235 and U-238. U-238 is the overwhelming majority (>99%) of the world's uranium, and is not fissionable (does not react). One cannot extract the U-235 chemically, only mechanically, by taking advantage of the minute difference in atomic weight. Therefore, extraction is extremely expensive and difficult. Uranium that has added U-235 (extracted from another block of mostly U-238) is called "enriched" uranium. To be used in a nuclear reactor, the uranium must be ~2.5-3.5% U-235. To be used in a nuclear weapon, 85% U-235 is required for a good weapon, 20% is required for a moderately usable weapon, and if you have enough of the actual material, you can go a little under 20%. In any case, reactor fuel is not even close pure enough to be used for a fission bomb, and other use in a bomb is not the traditional "nuclear" bomb, and doesn't cause nearly the same level of harm.
HOWEVER, if you put reactor fuel through a nuclear reactor, one of the waste byproducts is Plutonium. Plutonium is relatively easy to extract, much cheaper than U-235 and, most dangerously, fissions very well for nuclear bomb purposes. The Hiroshima bomb was a U-235 bomb, and I believe that every other bomb ever exploded (Nagasaki, all the test weapons, etc) are Plutonium weapons.
Thus, nuclear reactors are extremely dangerous in the wrong hands. Give Iran a nuclear power reactor, and it has access to Plutonium. It'd need to extract it, which isn't quite as easy as I've made it sound, but it's MUCH easier than extracting plutonium from no waste at all.
However, nuclear energy is a viable way to partially address some concerns around global warming, which can hit an unstoppable spiral once it hits a critical point (we don't really know what that level is, but we've got good reason to believe it exists)
Thus, the ability to create a reactor that DOESN'T spit out usable plutonium would be an incredible way to cheapen energy, reduce carbon emissions and still secure nuclear warfare.
So to finally get to my point, Israeli scientists have figured out a way to "spike" reactor Uranium with Americium that gets embedded in the Plutonium waste. Extracting it requires an additional method of processing, which is another step we can try and secure. This is very good news for black swan delayance/avoidance. http://dsc.discovery.com/news/2009/03/11/plutonium-bombs.html