Sometimes there are circumstances that you could not possibly account for when you are designing a product or reviewing it from the FDA standpoint. While a company may design a product to function a certain way under a certain set of conditions, chances are that someone somewhere using the product will use it in a way that was not thought of or intended, causing a failure or anything else that is cause for concern. From the development standpoint, there are only so many ways you can try to account for anything that could go wrong, and from the FDA standpoint, the same applies. If someone uses a product in a way that is away from the norm, or if the product is undergoing unusual circumstances, then there is a great chance that it will fail or not perform the way it was intended to perform.