A person injured by an allegedly defective product may not be able to recover damages if it is shown that he or she misused the product, or used it in a manner other than that which is expected. Products are generally designed to be used a certain way and serve a specific function. A manufacturer of a product is not liable if an individual is injured while using a product in a way other than the way it is supposed to be used. The key question is whether the misuse was foreseeable to the manufacturer, since manufacturers of products are required to recognize that some type of misuse by users of their products is expected, and thus the products are required to be designed to avoid foreseeable misuse. The defense of misuse is essentially a defense based on causation (and comparative negligence in non-strict liability negligence cases).
Example: A toaster oven is used to heat food, not warm mittens. If you use a toaster to warm mittens, and it subsequently causes a fire or causes serious burns to your hands, you cannot sue the manufacturer for a defective product because you didn't use the toaster the way it was meant to be used, or in a manner which would be foreseeable to the manufacturer.
If you substantially alter a product after you purchase it and the product then causes you physical injury, you generally cannot sue the manufacturer alleging that the product was defective.
Example: If you purchase a power saw that has a safety guard covering the blade to prevent injury to your fingers, and you remove the guard because you feel it makes the saw more difficult to use, if you continue to use the saw and cut one of your fingers off, the manufacturer would not be liable.
Control of the Defendant
In order to recover for injuries caused by a product, it must be shown that the product was defective at the time it left the control of the party against whom a claim is made. As such, in the case of a manufacturer, the product must have been defective at the time it was sold and delivered to a wholesaler. In the case of a wholesaler, that time would be when the product is sold and delivered to a retailer. With a retailer, that time would be when the product is sold and delivered to a consumer. In most design defect cases, the product is alleged to be defective at all times, since the theory is that a fundamental design flaw renders the product unreasonably dangerous- regardless of who’s hands it is in. Needless to say, if the condition of a product changes so as to render the product unreasonably dangerous after the product has left the control of a particular party in the chain of commerce, that party cannot be held liable for damages caused by the product, unless the change was reasonably foreseeable within the scope of the intended use of the product.