Every now and then you stumble across a piece of physics, some detail about how the universe works, that is extremely lucky in some sense. For example, it's very convenient that Si is a great semiconductor, and at the same time SiO2 is an incredibly good insulator - in terms of the electric field that it can sustain before breakdown, SiO2 is about as good as it gets. Another example is GaAs. While it doesn't have a nice oxide, it does have some incredibly nice crystal growth properties. I've been told that through some fortunate happenstance of growth kinetics, you can do growth (e.g., in a molecular beam epitaxy system - a glorified evaporator) under nearly arbitrarily As-rich conditions and still end up with stoichiometric GaAs. Somehow the excess As just doesn't stick around. A third example is the phase diagram of 3He/4He mixtures. Mixtures of the two helium isotopes phase separate at low temperatures (T below 600 mK) >3He-rich phase that's almost pure, and a dilute phase with about 6% 3He in 4He. If you pump the 3He atoms out of the dilute phase, more 3He atoms are pulled from the concentrated phase to maintain the 6% concentration in the dilute phase. There is a latent heat associated with removing a 3He atom from the concentrated phase. The result is a form of evaporative cooling: the temperature of the concentrated phase decreases as the pumping continues, and unlike real evaporative cooling, the effective vapor pressure of the 3He in the dilute phase remains fixed even as T approaches zero. This happy piece of physics is the basis for the dilution refrigerator, which lets us cool materials down to within a few mK of absolute zero.
Any suggestions for other fortunate, useful pieces of physics?