Shallow defects, by definition, do not constitute non-radiative recombination centers and are, therefore, mostly benign and of less importance in inorganic covalent semiconductors, such as silicon or gallium arsenide. Perhaps, for this reason, shallow defects have received comparatively less attention from the metal halide perovskite community. In this work, we observed through systematic experimental and computational studies that shallow iodine interstitial defects (Ii) can be unintentionally generated during commonly used surface treatments for state-of-the-art perovskite solar cells (PSCs). Although shallow and, thus, not particularly detrimental to initial device performance, Ii aggravates the phase instability of FAPbI3 perovskites to accelerate its degradation. A universal strategy is proposed to resolve this issue to significantly elongate the operational lifetime of PSCs.