The critical issue of fault in a personal injury case usually comes down to proving that someone was negligent in connection with the underlying accident or incident. And as part of establishing the negligence, the injured person must show that the at-fault party's action (or inaction) was a cause (sometimes called a "proximate" cause in legalese) of the resulting injuries.
Intervening and superseding causes—which occur when the action of a third party, or even an act of nature, play a role in causing the plaintiff's injury—disrupt this so-called "causal chain" and can diminish or even wipe out the defendant's liability, which means intervening and superseding causes can reduce or eliminate your ability to get fair compensation ("damages") for your injuries.
Let's say a person is getting off a bus in a parking lot. Meanwhile, the driver of a nearby car puts her vehicle in reverse and runs over the person who just got off the bus.
The bus passenger who got hit sues the bus company for failing to provide a safe place for passengers to disembark. But in this situation, the actions of the car driver may be considered a superseding cause of the passenger's injuries, whether or not the bus company could also be considered negligent. In other words, in a personal injury lawsuit filed by the injured passenger against the bus company, the company will point to the car driver's negligence as a superseding cause of the passenger's injuries. (Get the basics on negligence in a personal injury case.)
If, after the defendant acts negligently toward the plaintiff, a new cause combines with the defendant's negligence to contribute to or worsen the plaintiff's injury, that new cause is sometimes called an "intervening cause." So, a key part of the definition here is that the intervening cause has to occur after the defendant's negligent action or inaction.
An intervening cause can be the action of another person (who is generally called a "third party"), and it can also be an act of nature, such as a branch falling from a tree or a weather-related event. The original defendant will usually still be considered at least partially liable for the plaintiff's injury even when an intervening cause is said to exist.
This is the biggest difference between an intervening cause and a superseding cause. In contrast to an intervening cause, which does not relieve the original defendant of liability, a superseding cause usually does relieve the original defendant of liability. In other words, a superseding cause is an intervening act that is legally sufficient to transfer blame for the harm in question from the defendant to a third party, or to a natural event.
The key difference between an intervening cause and a superseding cause is foreseeability. An intervening act will be called a superseding cause (or act) that relieves the original defendant of liability when the intervening act was or should have been reasonably foreseeable to the original defendant.
Let's look at an example of this. Let's say that a homeowner digs a hole into a sidewalk and negligently leaves it open without any warning to pedestrians. A fellow pedestrian negligently fails to leave enough room for the plaintiff to pass on the sidewalk, and the plaintiff falls into the hole. Under longstanding premises liability standards, the homeowner is almost certainly negligent for leaving an open hole in the sidewalk. The second pedestrian was also likely negligent for crowding the plaintiff into the hole.
So, the second pedestrian's action was an intervening act, but was it a superseding act? The answer is probably no because the intervening action was (or should have been) reasonably foreseeable to the homeowner. If you leave an open hole in a sidewalk, it's reasonable to foresee someone falling into it, especially if the sidewalk is crowded. Learn more about proving fault in a personal injury case.