In Kurt Vonnegut's novel Cat's Cradle, a scientist develops a polymorph of ice with a very strange property.
Unlike ordinary ice, that melts at temperatures above 0 C, the new form -- "ice-nine" -- melts at 45.8 C, above the temperature experienced anywhere but the hottest places on Earth's surface. Even worse, a tiny bit of ice-nine acts as a seed crystal, converting any ordinary water it comes into contact with into more ice-nine. Not only is it rapidly (nearly instantaneously) fatal if ingested, it is capable of wiping out all life on Earth if any is introduced into bodies of water.
While this is a science fiction scenario, there is some real science behind it. Materials are stable when they are in a "potential well," a form that is the (locally) lowest energy state. The situation changes, though, when something alters the energy required to overshoot the next highest "hill" in the thermodynamic landscape and allows whatever-it-is to achieve an even lower-energy, and thus more stable, state.
Something like this is what happens with prions, the misfolded bits of protein that are responsible for mad cow disease, Creutzfeldt-Jakob disorder, and other "spongiform encephalopathies." The contagion occurs because the misfolded version of a protein called PrP is not only more stable than the one with the correct conformation, it triggers an ice-nine-like reaction when it comes into contact with normal PrP; a pair made up of one normal molecule of PrP and one misfolded one is intrinsically less stable than two abnormal ones, so it gradually converts the PrP in the brain into tangles of misfolded protein.
Not fatal as quickly as ice-nine, but still fatal.
This same idea crops up elsewhere. You may have heard some talk about the possibility that the universe is in a "metastable state" -- a "false vacuum" that is, like ordinary water in Vonnegut's novel, only stable because it's in a local thermodynamic trough, but (given the right conditions) could be nudged up and over a hill into a much more stable state. A "true vacuum." If this happened, it would release so much energy that it would trigger neighboring regions into surmounting the hill and falling into the true vacuum state themselves, and on and on it would go, propagating outward at the speed of light and destroying everything in its wake. The conversion would happen so quickly that if it swept past you and hit your feet first, the neural signal saying that your feet had been disintegrated wouldn't even have time to reach your brain before the rest of you disintegrated, too. Which, honestly, wouldn't be a bad way to go. No warning, not even the briefest moment of panic, just... poof.
There's one other example like this I know of, which comes from the realm of particle physics. In 1950, a particle called the lambda baryon was discovered by a team at the University of Melbourne, and given its relatively high mass, it was unexpectedly stable -- decaying in one ten-trillionth of a second and not the predicted one hundred-sextillionth. The team called this property strangeness, but it wasn't explained until 1968, when the quark model finally received experimental confirmation, and the lambda baryon was shown to be made of one up, one down, and one strange quark, an unusually stable configuration.
Its makeup exempts the lambda particle from the baryon version of the Pauli Exclusion Principle, which states that two or more particles with half-integer spins can't occupy the same quantum state. And this is where things get interesting.
Initially, it was thought that all strange particles eventually decay into particles composed only of up and down quarks (the lambda can do this two different ways -- either into a proton and a negative pion, or into a neutron and a neutral pion). They lose their "strangeness." But the brilliant physicists Arnold Bodmer and Edward Witten have shown that this isn't always so -- that in larger assemblages of quarks, the most stable state is one with equal numbers of up, down, and strange quarks, which (like the lambda) would be immune to the Pauli Exclusion Principle, and thus could release energy by collapsing into (much) smaller volumes.
They called these assemblages strangelets.
And much like my previous examples, this release of energy could trigger the conversion of normal matter nearby into more strangelets, and the whole thing would spread. It's been suggested that this might be the ultimate fate of any neutron star that continued to gain more mass. The gravitational force would eventually rise to the point that the core would no longer have the capacity to support its own weight, and would release that energy in the most convenient way -- by converting to strange matter.
Like ice-nine, prions, and the true vacuum catastrophe, once that conversion happened, it'd be pretty much stuck that way. There's no easy way out of the lowest local potential well. In this case, though, the conversion would be limited to the neutron star; there'd be no mechanism for it to spread through the near-vacuum of space to the rest of the cosmos, which is good news for us. It also bears mention that the hallmark of such "strange stars" suggested by Bodmer and Witten -- extremely high rotation rate, because of conservation of angular momentum as the strange matter at the core collapsed into a smaller volume -- has not been observed.
So it may well be that the Bodmer/Witten model for strange matter is flawed, and like the lambda baryon, anything containing strange quarks ultimately decays into ordinary matter. Or conversely, perhaps some of the weird and unexplained behavior of astronomical objects is because they're strange, both in the technical and the vernacular sense of the word.
Either way, it's probably best if we stay right here in our nice, comfortable local well of stability. None of the other options I've read about sound like all that much fun.
