There’s a lot of people who’ve said that missing a milestone is a failure, but it’s not just that, it’s your failure. I don’t believe this is always the case. Is it technically a failure? Of course. However, there will always be situations that are out of your control and all you can do is have planned for it and set another milestone in order to get back on track, even if it takes more work than usual on your part. As long as the project is back on track by the next few milestones, or at the very least close to it, and you finish on time then you can consider it overall a success. The only “learning opportunity” from this would be planning for such an eventuality at the start of the project. Having milestones with more leeway allows you to counteract such pitfalls. Finishing milestones early never hurt anything, but finishing them late is an issue. Planning requires you to have the foresight to see such eventualities and if you can work them out before they happen you’ll be a more effective engineer/manager.