Finite-temperature stability of crystals is of continuous importance in solid-state chemistry with many exciting properties only emerging in high-temperature polymorphs. Currently, the discovery of new phases is largely serendipitous due to a lack of computational methods to predict crystal stability with temperature. Conventional methods use harmonic phonon theory, but this breaks down when imaginary phonon modes are present, and anharmonic methods are thus warranted to describe dynamically stabilised phases. We investigate the high-temperature tetragonal-to-cubic phase transition of zirconia based on first-principles anharmonic phonon theory and molecular dynamics simulations as an archetypical example of a phase transition involving a soft phonon mode. It is shown that the stability of cubic zirconia cannot be attributed solely to anharmonic stabilisation, and is thus absent for the pristine crystal. Instead the stabilisation is attributed to spontaneous defect formation which is also responsible for superionic conductivity at elevated temperatures.