A Sprint may be cancelled when the Sprint Goal becomes obsolete. This is because:
The Sprint Goal is a short-term objective that provides guidance and focus to the Scrum Team throughout the Sprint. It is a flexible and negotiable commitment that can be adjusted as more is learned throughout the Sprint.
The Sprint is a container for all other Scrum events and activities. It is a time-box of one month or less during which a “Done” Increment is created that meets the Sprint Goal. The Sprint has a consistent duration throughout a development effort and only changes duration between Sprints.
A Sprint may be cancelled before it is over by the Product Owner if they determine that the Sprint Goal is no longer valid or valuable. This may happen due to various reasons, such as a significant change in the market, technology, business direction, or customer needs.
A cancelled Sprint should be rare and exceptional. It implies a waste of time and resources that could have been spent on delivering value. It also disrupts the rhythm and flow of the Scrum Team and the stakeholders.
When a Sprint is cancelled, any completed and “Done” Product Backlog items are reviewed and potentially released. Any incomplete Product Backlog items are re-estimated and put back on the Product Backlog. The Scrum Team then plans for a new Sprint.
Other options, such as when the Developers determine the product plan is infeasible, when it becomes clear that not everything will be finished by the end of the Sprint, or when the sales department has an important new opportunity, are not valid reasons for cancelling a Sprint. They may reflect a misunderstanding of what a Sprint Goal is or how Scrum works.
References:
[Scrum Guide], page 15, section “Sprint Goal”
[Scrum Guide], page 9, section “Sprint”
[Scrum Guide], page 16, section “Cancelling a Sprint”
