Supercritical shear occurs when the shear stress exceeds the strength of the perfect crystal. An estimate of the theoretical strength of the molecular crystal pentaerythritol tetranitrate is compared to experimental observations of the shock response of this explosive. It is concluded that supercritical shear may be occurring at shock stresses of several GPa in underdriven shocks where there is an elastic wave followed by a slower plastic wave. The induced simultaneous slip on sterically crowded cystallographic planes may be the mechanism for shock initiation of detonation in this explosive, at least in the low stress regime.