If the injured user was using the product in a manner not intended by the manufacturer or retailer, or had altered the product so that safety features were disabled, it may not be possible to prove that injuries were caused by defects in the product. The defendant may be able to successfully claim that the injuries were caused by the acts of the plaintiff.

Questions of negligence and breach of warranty are also grounds for a claim for damages under product liability. In the case of negligence, if it can be shown that a company was negligent in testing its product adequately or in supplying directions for its use, the injured party probably has grounds for filing suit. Similarly, a manufacturer implies a warranty for fitness of use and freedom from defect when an item is sold. If the item proves to be defective, or is unfit for the purpose intended, an injured user can file a product liability case. In a case of negligence, it is important that the plaintiff be able to show that the product was defective when it left the control of the party he is suing. It is not possible to hold someone liable for a defect that occurred after that party had control over the product.

In product liability, there are various areas of defects that can occur. If a claim of strict liability is to be pursued, the injured party will need to show that the product was unreasonably dangerous for its intended use, due to a defect. There are generally three areas in which a product can be unreasonably dangerous:

The manufacturer or seller can fail to warn about angers associated with the products use. Manufacturers and sellers are expected to give adequate warnings about possible dangers, and to provide clear and adequate instructions of use. Failure to do so can cause a useful product to become deadly. For instance, coolants used in automobiles are extremely toxic – failure to print warnings of this toxicity on the product labels could lead to an accidental poisoning and thus to a suit for damages.

The product may have a design defect. This means that the product is manufactured with a defect, even if it is assembled perfectly. An example would be a car gasoline tank that is designed with weak walls such that an impact can rupture the tank and cause the car to catch on fire, even when the tank is correctly assembled and installed.

A manufacturing defect exists when an otherwise safe product is rendered dangerous because it is assembled improperly. A car whose wheel is installed with missing or cross-threaded bolts may lose a wheel at high speed, injuring or killing the driver and passengers. If it is proved that the car was manufactured with that defect, the manufacturer will be liable.

In all cases of product liability, it is essential to the success of the case that the product be preserved and that all paperwork showing the origin of the product be made available. Receipts showing purchase, any repair records, etc., can be vital to building a successful case.