Most of the paradigms in materials science and technology firmly rely on equilibrium materials, i.e., phases with the lowest Gibbs energy under thermodynamic conditions. In nature, however, complexity rules, and the other aspect is metastability. This review aims to provide a coherent description of such effects and to formulate hypotheses on the fundamental energetic mechanisms of residual metastability in crystalline metal oxide polymorphs. The authors especially focus on different forms of broken symmetry at the interface, e.g., induced by defects, lattice strain/stress, and elemental diffusion. The discussion also provides perspective on the technological implications and possibilities arising from identifying a general methodology for establishing metastability in polymorphic functional metal oxides for energy applications, catalysis, and electronics.