When a third person intervenes between the defendant's conduct and the plaintiff's injury, liability turns upon whether the intervening act is a normal or foreseeable consequence of the situation created by the defendant's negligence.
A superseding act breaks the causal nexus of proximate cause. An intervening act may well be superseding if it is extraordinary under the circumstances, not foreseeable in the normal course of events, or independent of or far removed from the defendant's conduct.