In Expert Mode, Skeletron's head is more resistant to damage until its hands are destroyed. Once one hand is destroyed, it will start shooting homing skulls at the player. It is also much faster. Its hands inflict the Slow debuff, and its head inflicts Bleeding. If the player gets caught by its head-spin attack, Skeletron can "stun-lock" them, a lethal situation that can only be escaped by high mobility, e.g. with a hook.