I think that Flaw, as in a Flawed diamond, describes an inherent problem well in fewer words.
Inherent is a good word to describe the flaw, but you didn't say if the flaw was of design or manufacturing. If you're attempting to get redress, it's important to distinguish a "manufacturing flaw" (they messed up making it) as opposed to a design flaw (this item was poorly conceived). The former is much more likely to get immediate redress, but the second at best will cause a new model or version.
If the product is software you can also use the word "bug" to describe any error that occurs.