A SCRUM approach was recently used at our organization to scope and implement a major system development and data migration initiative.
Sprint 5, preparation of migration data, resulted in major failure. Data was dirty; inaccurate and inconsistent, needed to be cleaned up but wasn't.
Despite a failed sprint that would directly impact all subsequent sprints, it was decided to continue.
Months later, the overall project goals were no met (because of sprint 5). The plan is to start the whole process over from sprint 5.