Anomalous preservation is the well-established but little-understood phenomenon of a long-term stability of gas hydrates outside their thermodynamic field of stability. It occurs after some initial decomposition into ice in the temperature range between 240 and 273 K. In situ neutron diffraction experiments reveal that the low-temperature on-set of this effect coincides with the annealing of stacking faults of the ice formed initially. The defective, stacking-faulty ice below 240 K apparently does not present an appreciable diffusion barrier for gas molecules while the annealed ordinary ice I_h above this temperature clearly hinders gas diffusion. This is supported by further experiments showing that the so-called ice I_c formed from various high-pressure phases of ice, gas hydrates or amorphous ices does transform fully to ordinary ice I_h only at temperatures near 240 K, i.e. at distinctly higher temperatures than generally assumed. In this light, some quite disparate observations on the transformation process from ice I_c to ice I_h can now be better understood. The transformation upon heating is a multistep-process and its details depend on the starting material and the sample history. This ‘memory’ is finally lost at approximately 240 K for laboratory time-scale experiments.