Product liability law varies from jurisdiction to jurisdiction, but it typically is a strict liability tort, meaning that the plaintiff does not have to show negligence on the part of the defendant in order to recover for the harm suffered. A product liability claim must only show that the defendant suffered some harm from a product that contained a defect at the time it left the control of the manufacturer or seller, as long as the seller is someone who is a regular seller of the product in question. Additionally, the plaintiff will have to show that he or she was using the product in a manner that is reasonably foreseeable for a purchaser of that particular product. Therefore, a product liability defense will require that the defendant show that one of the above elements is not present in the event that resulted in the harm.
A simple product liability defense will assert that the product was not defective at the time that it left control of the defendant. For example, the plaintiff may purchase a motorcycle and modify the engine to make it more powerful, and in the course of modifying the engine, he or she may have caused the defect that resulted in the injury. The defense will argue this very point, which, if believed by the jury, will result in exculpation of the defendant for the harm suffered by the plaintiff. The fact that the engine was modified does not automatically result in an effective product liability defense, however. Rather, the plaintiff may still show the defect that resulted in the harm was unrelated to his or her modification of the engine.
Another product liability defense the defendant may employ is that the plaintiff was using the product in an unforeseeable manner. The important consideration here is that the manner in which the product was used when the harm occurred was not merely atypical, but completely unforeseeable. Continuing the motorcycle example, if the plaintiff is harmed by a defect in the motorcycle while riding it off-road, it probably will not amount to an effective product liability defense. Although the motorcycle is intended to be driven on paved roads, it is clearly foreseeable that a consumer might bring it to an off-road path.
An example where this particular product liability defense may be used effectively is easy to imagine. If the plaintiff was harmed by an electric carving knife while using it trying to cut firewood, then the defense is more likely to apply. It is probable that a court would find it unforeseeable that a consumer would purchase an electric carving knife for use on a tree.