Helium-3's Phase diagram shows that at the right temperature and pressure combination, the solid region dips downward as temperature increases. That means that you can heat it up and it will freeze. What is going on?
Amazingly, below 0.3K the latent heat of fusion of $^3$He is negative i.e. you need to inject heat to make the solid freeze. This is the basis of Pomeranchuk cooling.
This is miles outside my area of expertise, but I believe it's because in the liquid you get large scale alignment of the spins called para magnons, and these lower the energy of the liquid relative to the solid.