Semiconductor nanowires have increased the palette of possible heterostructures thanks to their more effective strain relaxation. Among these core shell heterostructures are much more sensitive to strain than axial Ones. It is now accepted that the formation of misfit dislocations depends both oh the lattice mismatch and relative dimensions of the core and the shell. Here, we show for the first time the existence of a new kind of defect in core shell nanowires: cracks. These defects do not originate from a lattice mismatch,(we demonstrate their appearance in an essentially zero-mismatch system) but from the thermal history during the growth of the nanowires. Crack defects lead to the development of secondary defects, such as type-I-1, stacking faults and Frank-type dislocations. These results provide crucial information with important implications for the optimized synthesis of nanowire-based core-shell heterostructures.