When a milestone is missed, it means that something pertaining to the project was misplaced. This could be resources, foresight, proper documentation, or failure to plan for setbacks. If the missed milestone points out a deficit in the product that would not otherwise have been known, it is a project success. If a missed milestone was made up in time to not delay the project, it is a project success. Despite this, I believe that at its core, missed milestones represent a failure on some managerial level and that constitutes them as manager failures. So, while some missed milestones may be successes in terms of the product, all of them are failures for the management.