Our new study shade new light on why magnesium, a promising lightweight metal, suffers from limited ductility. Using in-situ TEM and atomistic simulations, we discovered that <c+a> dislocations—initially key to plastic deformation—quickly become inactive, triggering dislocation avalanches and twinning. The culprit? Stacking faults that block further dislocation motion, offering crucial insights for designing tougher, more formable magnesium alloys. The manuscript is now published: Jeong et al., Nanoscale Mechanisms Limiting Non-Basal Plasticity in Magnesium, Acta Materialia 296 (2025) 121261.