If you're truly agile, there's no harm in missing user stories. Your business people, including customer and stakeholder representatives, are working closely with the development team. You are regularly delivering working software that the stakeholders or stakeholder representatives can use and provide feedback on. You are responsive as the requirements change (or you identify things that you've missed), using the short delivery cycles as feedback loops.
A missing (or incomplete or wrong) requirement, in any format, shouldn't be missing (or incomplete or wrong) for long. If, for any reason, you aren't getting rapid feedback from stakeholders or stakeholder representatives, the delivery team and delivery organization should be reflecting on how to become more effective - how to identify what is most important, how to deliver those most important pieces of functionality quickly, and how to elicit and respond to feedback on the delivered functionality - on a regular basis.