Solid electrolytes are advocated for next-generation lithium batteries given their conformity against lithium metal anodes. Intuitively the mechanical stiffness of the solid electrolytes should discourage irregular lithium deposition. However, often lithium dendrites are found to form in solid electrolytes. We find that the solid electrolytes defy the conventional wisdom of electroplating. An irregular lithium dendrite leads to a misfit strain which alters both the reaction kinetics and transport in the solid electrolyte. These competing mechanisms give rise to an interfacial instability, which marks the onset of dendrite formation. Here in we discuss the regimes to promote uniform deposition.