If the product that injured the customer was thrown away, it may be difficult to prove the defect in the product. However, there are a few ways to prove the defect in a product that injured a customer. For example, if the injury occurred because of a design defect, there may be evidence of this in the design plans for the product. If the injury occurred because of a manufacturing defect, there may be evidence of this in the manufacturing records.
That being said, if the problem was specific to this product then you’ll have no way of proving that one of the products on the line was defective in some way. This is especially relevant when it comes to foods.