A defect in manufacturing is one that the manufacturer did not intend. A manufacturing defect is the clearest instance in which strict liability applies. Under the Restatement (Third) of Torts: Products Liability, a product "contains a manufacturing defect when the product departs from its intended design even though all possible care was exercised in the preparation and marketing of the product."

An example of a manufacturing defect would be a car's braking system that does not work properly and causes a plaintiff to have an accident. Even though the manufacturer of the car did not intend for the brakes to malfunction, and even though the manufacturer was not negligent in the design of the brakes, the strict liability doctrine in products liability law could render the manufacturer liable.

A plaintiff may have difficulty proving that a product caused the plaintiff's injuries. For example, even if a car had some defect in the braking system, the driver's poor reaction to driving conditions may have been the actual cause of an accident. Additionally, in some circumstances, it may be difficult for a plaintiff to prove that a defect caused an accident due to the damage to the product. A car may be so heavily damaged in an accident, for instance, that it is impossible to prove what caused the accident to occur.

In some instances, a plaintiff can rely on the "malfunction doctrine" to prove causation. Under this doctrine, if the circumstances of an accident indicate that a defect caused the accident, and the plaintiff can produce evidence that removes other possible causes, then the plaintiff can prove causation even if the product is damaged or destroyed. This doctrine is similar in application to res ipsa loquitur in the law of negligence.