In Software Development we are always working on providing as much Value as possible against the lowest cost possible. SCRUM was designed to implement this for the Software Development process. SCRUM does not say anything about the process that leads to a full Product Backlog. It simply states that "This should be done or else you can't develop anything".
In one of my projects, the time to market was not sufficient to build a full "Ready" Product Backlog before starting development. This is a problem. If we'd wait to start development until the full backlog is available, we'd miss the marketing window of opportunity and thus the value of the product would be greatly decreased. Going back to "Greatest value against lowest cost", missing the window would greatly diminish the left side of the equation.
And so the decission was made to use a "Just in time" approach for the full product backlog. The solution for this specific project lay in SCRUM: Instead of putting all the Product Owner responsibility on one person, a Product Owner SCRUM was installed. The people that were responsible for detailing the items on the Product Backlog were joined into a SCRUM and started working the same way the software development team did their work; in 2 week sprints, with daily standups, their own Product Backlog (consisting of Product Backlog items that would deliver "Ready" product backlog items for the Development team).
The biggest challenge we faced was velocity. If the velocity of the Product Backlog team proved insufficient to fill the Ready Product Backlog of the Development team, the development team's efforts would grind to a halt. To this end, the backlog was filled with a number of "Nice to have" features that could be realised if and when the Must have features weren't being delivered as Ready quickly enough. These "Nice to have" features included things like automated installation, extensive automated test scripts (on a platform, not in a vaccuum), reporting, analysis and monitoring tooling. All of those were basically future features we'd like to do.
In the end the Product Owner team delivered on schedule and the entire process went quite smoothly. Since this product was meant to be developed as a contingency measure for a different project, our product never saw production. But the entire process was an eye opener for the rest of the company.