If a milestone is missed I would consider it as failure on the initialization and planning stage of the project cycle. If a milestone is missed especially on the red path which THE most important path for the project cycle it would be a major back log and tough one to recover from in order to complete the project on life cycle.
This can be avoided by understanding the overall critical impact of the step and proper planning of the the whole step and taking care during implementation and control step of the project cycle. But even if the project is a failure proper implementation of at least the closing step in the cycle would ensure that same mistake would not be repeated in the future projects and everyone would learn from the mistake.