Agile software development methods assume the significant amount of uncertainties and risks in the software development. The idea of the iterative development  employed by agile methods is aimed at verifying the project direction and state frequently.

Agile risk mitigation

Agile approach at risk mitigation is to attack the most unsure items first and therefore lessen the amount of the unknown as early as possible. In a way every iteration planning is a risk analysis and mitigation planning session. Imagine a project, where supporting millions of users is something the team is not sure is possible. In this case agile methods advocate for implementing the scalability support before the majority of the features.

One of the corner stones of the agile methods is transparency. Working software over the documents is one of the primary agile values. All the agile methods are explicitly based on making things explicit and clear.

Release and monthly level

There is always some form of a product backlog present with very clear priorities that leave no chance for listing all items as "absolutely mandatory". Product owner or customer have to make hard decisions and throw away something old if they want to do something new. There is not or at least should not be any way to silently sneak he extra work in. Teams compute their velocity per iteration and together with the product backlog it allows the stakeholders for seeing the real project status very early, allows for actually making the decisions on the release content and schedule without having to force the team work overtime.