Studying defect formation and evolution in MethylAmmonium lead Iodide (MAPbI 3) perovskite layers has a bottleneck in the softness of the matter and in its consequent sensitivity to external solicitations. Here we report that, in a polycrystalline MAPbI 3 layer, Pb-related defects aggregate into nanoclusters preferentially at the triple grain boundaries as unveiled by Transmission Electron Microscopy (TEM) analyses at low total electron dose. Pb-clusters are killer against MAPbI 3 integrity since they progressively feed up the hosting matrix. This progression is limited by the concomitant but slower transformation of the MAPbI 3 core to fragmented and interconnected nano-grains of 6H-PbI 2 that are structurally linked to the mother grain as in strain-relaxed heteroepitaxial coupling. The phenomenon occurs more frequently under TEM degradation whilst air degradation is more prone to leave uncorrelated [001]-oriented …