Failing to meet the milestone is in sense considered a failure. However, there should always be a contingency plan if you fail a milestone. Currently, I am taking capstone and we have missed a few milestones, however as long as it doesn’t effect the overall project by a lot, it shouldn’t be a big problem. In addition, you should always give yourself a little extra time so that you don’t miss your milestones. For example, there are many times where you order something from eBay or other websites and it could be delayed by few days. You as a project manager should always think about this.