Scope can be renegotiated with product owner

The full paragraph from the scrum guides sprint goal section is as follows: 

The Sprint Goal is created during the Sprint Planning event and then added to the Sprint Backlog. As the Developers work during the Sprint, they keep the Sprint Goal in mind. If the work turns out to be different than they expected, they collaborate with the Product Owner to negotiate the scope of the Sprint Backlog within the Sprint without affecting the Sprint Goal.

During sprint planning, the developers select the PBIs from the PB that will help the team achieve the sprint goal.  As more is understood about the PBIs during the sprint, the Developers may find that a PBI requires more work than initially thought which could prevent them from completing the PBI by the end of the sprint.  If this is the case, they can discuss this with the PO in order either decompose it further in order to try and retain some slices of it in the sprint that will still move them towards the sprint goal, or possibly remove it from the SB altogether, in order to prevent the sprint goal from becoming obsolete. 

Suppose you're developing a vaccine for a dangerous virus. It's a complex challenge. Even as you develop and evaluate different vaccine types and accelerate them into production, the virus is mutating into new forms. The scope of the work is changing as you learn more about the problem and the solution that is required.

The Product Owner is accountable for the vaccine you produce in terms of the value it delivers. The scope of work therefore has to be clarified and renegotiated with the PO as more is learned. Both the items on the Product Backlog -- and those of them which are planned in the current Sprint Backlog -- are subject to emergence, learning, and change.

Thanks Scott and Ian!