A product platform is a set of subsystems and interfaces that are commonly shared by a variety of products. Although platform sharing is considered an effective means of cost saving, it also runs the risk of propagating a particular failure across multiple products when the platform is defectively designed. Thus, sharing a common platform effectively can amplify the risk of incurring a large number of failures. In this paper, we formulate a quantitative model for assessing amplified failure risk. Our analysis shows an unexpected result that the platform risk increases as we become more assured of our design capability; i.e., when a defective design rarely happens, the magnitude of risk amplification becomes much larger. The numerical investigation of a platform planning case sheds light on the importance of risk assessment in determining the level of commonality in designing a platform.